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

Jack-of-all-trades:
A person who knows how to do a lot of different things.

Jack up (v.): To increase (prices, etc.) by a high margin. Example: "The street vendors jack up their prices on holidays."

Jam (n.): Predicament, problem. Example: "I'm in a bit of a jam here. Do you think you can help me?"

Jam-packed:
Very crowded. Example: "The stadium was jam-packed on Saturday."

Jiffy: In a jiffy = Very quickly = In no time

Jinx (v.): To bring bad luck or failure by saying something. Example: "Stop saying that! You're gonna jinx me."

John (n.) : Bathroom, toilet, can.

(To) jump all over someone/ jump down someone's throat:
To criticize or blame someone. Example: "As soon as I told her I was going out with my friends, she jumped all over me."

(To) jump the gun:
To get something started too soon; to start too soon. Example: "We have to do a lot more work before we launch this product. We don't want to jump the gun."

(To) jump at:
To quickly accept. Example: "The journalist jumped at the chance to interview Ryan Gosling."

Jump ship (v.): To leave one's job suddenly, usually to go work somewhere else.

Junkie (n.): Addict.

Just about:
Almost. Example: "I'm just about finished."

Just plain (adj.): Clearly; simply. Example: "He's just plain stupid."

Just now:
Just a minute ago. Example: "He called me just now to tell me he was going to quit his job."

Just what the doctor ordered:
Exactly what is needed. Example: "This vacation is just what the doctor ordered."