Here is our list of American idioms that start with "B":

Bachelor pad:
An apartment where a single man or woman (sometimes referred to as a "bachelorette pad") lives alone.

Back in the day:
Many years ago; a long time ago. Example: "Back in the day, we used to pay much more for laptops."

(To be) back on one's feet:
(To) recover (from an illness). Example: "You'll be back on your feet in no time."

Back to the drawing board:
A phrase that is said when you have to start some process from the beginning (usually after having failed in the previous attempt). Example: "Well, that didn't work... Looks like it's back to the drawing board."

Backhanded compliment:
An insult disguised as a compliment. Example of a backhanded compliment: "Hey, don't feel bad! You're not as fat as you used to be!"

(To) badmouth/bad-mouth:
To verbally criticize; to knock; to talk badly of. Example: "She's such a negative person. All she does is badmouth people all day."

Baggage (n.):
Short for "emotional baggage", which is a collection of painful memories, experiences, mistrust, etc. carried around as a result of negative past experiences/relationships. Example: "I wouldn't get involved with her. She's got a lot of baggage."

Behind bars:
In jail. Example: "The robbers were caught and are now behind bars."

(a) ball-park/ballpark figure:
A rough estimate. Example: "Can you give me a ball-park figure of what this project will cost?"

(To get/jump on the) bandwagon:
To begin to like something/start doing something because it's popular, "hip", or everyone else is doing it. Example: "Everyone's jumping on the tablet bandwagon."

(To) bank on something:
To count or rely on something.

(To) bark up the wrong tree:
To ask the wrong person; to make the wrong choice. Example: "The gangster told the cops they were barking up the wrong tree in thinking he was responsible for the robbery."

Bash (n.):
Party.

(A) basket-case:
A very nervous person; someone at the verge of being neurotic. Example: "All the stress from the divorce turned John into a basket case."

(To) be a fan of someone/ something:
To like, idolize, admire someone/ or something. Example: "I'm not a big fan of heavy metal music."

(To) be in one's element:
To be completely comfortable doing something; to do something that comes very naturally to someone. Example: "When it comes to speaking in public, the senator is in his element."

(To) be the case:
To be a reality. To be the way things are. Example: "I wish I had a million dollars, but that's not the case."

(To) be up to no good:
To be planning something bad, mischievous, etc. Example: "I could tell from the look in his eyes that he was up to no good."

Beat (adj.):
Tired. Example: "Man, I'm beat. I'm going to bed."

(To) beat around the bush:
To avoid getting to the point. Example: "Stop beating around the bush and tell me what you really think."

(To) beg to differ:
A polite way of saying "to disagree", most often heard in the phrase "I beg to differ!"

(To be) behind (someone):
To be in the past. To no longer be the case. Example: "I used to smoke, drink, and take drugs, but all that is behind me now."

(To get) behind (someone/something):
To support (someone/something). Example: "I'm behind you on this."

Believe it or not:
Used at the beginning of a sentence to state that something is true whether one chooses to believe it or not. Example: "Believe it or not, I still care for her."

Bent out of shape:
Upset/agitated. Example: "Don't get all bent out of shape over that."

(To give someone a) big hand:
To applaud (for someone) energetically. To give (someone) a big round of applause. Example: "Let's give our performers a big hand!"

(To have a) big mouth:
To not be able to keep a secret. Example: "Don't tell her anything. She's got a really big mouth."

Big-shot (noun/adjective):
An important person. Example: "All the big-shots at headquarters never listen to what we have to say."; "A big-shot reporter."

(A) Bimbo:
A foolish or empty man or woman. Example: "John only talks about his car and his clothes. He's a real bimbo."

(The) birds and the bees:
Sex; human reproduction. Example: "It's about time I talked to my son about the birds and the bees."

(A) bite to eat:
A snack, some food. Example: "Let's go grab a bite to eat before we go to the game."

(To) bite the hand that feeds you:
To do harm to someone who helps you.

(To) bite one's tongue:
To struggle not to say something that you want to say. Example: "I wanted to tell her the truth, but I had to bite my tongue because I had promised Bill that I wouldn't say anything."

Black sheep (of the family):
The worst or least accepted member of a family. Example: "Peter is the black sheep of his family."

(A) blast:
A great time; a fun time. Example: "We had a blast last night."

(To) blow:
To lose is a wasteful way/ to waste. Example: "Sarah blew all her money on gambling."

(To) blow (someone) away:
To really impress (someone). Example: "We were all blown away by her performance."

(To) blow someone's cover:
To reveal someone's secret, or true identity. Example: "The spy was very careful not to blow her cover."

(To) blow (something) out of proportion:
To make (something) seem much more important than it actually is. Example: "Of course his reaction was extreme! He always blows things out of proportion."

(To) blow (someone) out:
In sports, - to win by a large margin, by a lot of points. To be blown out means to lose by a large margin. Example: "Q: How did your team do? A: They were blown out."

B.O.:
Short for "body odor" - the foul smell from one's armpits, especially when one doesn't shower/wash. Example: "The woman sitting beside me had really bad B.O."

(To) bolt (for):
To run (towards). Example: "As soon as I turned around, he bolted for the door."

Booze:
Alcohol.

(to) boss (someone) around: To be bossy towards (someone); to constantly tell (someone) what to do. Example: "It's annoying how she bosses everyone around."

Break (n.):: Stroke of luck. Example: "We got a big break in the case today!"

(To) break even:
To neither win nor lose. Example: "Michael thought he would lose $200, but he ended up breaking even."

(To) break it up:
To end a fight. Example: "All right, break it up, you two!"

(To) break new ground:
To do something that hasn't been done before; to innovate. Example: "Dr. Davis was breaking new ground in cancer research."

(To) break someone's heart:
To cause someone (strong) emotional pain. Example: "Fiona broke James' heart when she refused to marry him."

(To) break the news to someone/ to break "it" to someone:
To tell someone some important news, usually bad news. Example: "I hate to be the one to break it to you, but your son has been selling stolen baseball cards."

Broke (adj.):
Having no money. Example: "He should really get a job. He's always broke!"

Bum (n.):
Mendicant; Homeless person.

Bummed (adj.):
Disappointed; Dejected. Example: "He was really bummed about having lost the game."

'Burbs (n.pl):
Short for "suburbs"; Example: "Tina lives in the 'burbs with her folks."

(To really) burn someone up:
To really upset someone. Example: "It really burns me up when he talks to me in that tone of voice."

Burnt out:
Exhausted from doing something too much and too intensely (especially used when speaking about work, etc.) Example: "He was completely burnt out after working for three weeks straight."

(To) burst into tears:
To start crying suddenly. Example: "She burst into tears when she heard the news."

(To) bust:
To nail; To catch; Example: "He got busted for driving drunk."