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

Icky (adj.): Gross, disgusting.

If worse (or worst) comes to worst:
If things get really bad. Example: "If worse comes to worst, we can always fire him."

Iffy (adj.): Uncertain. Example: "That's a little iffy at this point."

Ill-at-ease (adj.):
Uncomfortable. Example: "She seemed to be ill-at-ease talking to the doctor about her problems".

In a bad way:
Not well; in bad shape. Example: "He was in a bad way, partly because of all the alcohol that he drank daily."

In bad taste:
Rude. Vulgar. Obscene. Example: "John's jokes are always in bad taste."

In a bind: In a predicament; in a difficult situation. Example: "I'm in a bit of a bind. Do you think you can help me out?"

In broad daylight:
Publicly visible in the daytime. Example: "This city is very dangerous. You can get robbed in broad daylight."

In good shape/ condition:
Physically and functionally sound and sturdy. "Shape" is generally used more for people. Example: "The car is in good condition. Bill is in good shape."

In mint condition:
In perfect condition (not used for people). Example: "This baseball card is in mint condition."

In no mood to do something:
To not feel like doing something; to not want to do something. Example: "I'm in no mood to cook dinner tonight."

In no time: Very soon. Example: "Don't worry. We'll be there in no time."

In a row: One after another; Straight. Example: "Our team has won five games in a row."

In season:
Currently available for selling (often said of fresh fruit and vegetables). Example: "Tomatoes are very cheap now because they're in season."

In stock:
Available for purchase, as in goods in a store. Example: "P1: Do you have any more of these books in stock? P2: No, I'm sorry we don't."

In the air:
Everywhere. All around. Example: "Love is in the air, everywhere you look around."

In the cards: Likely (to happen). Example: "A promotion is not in the cards for him." (He most likely will not be promoted).

In the doghouse: In trouble. To be in the doghouse = To be in trouble.

In the long run:
Over a long period of time; ultimately. Example: "He smokes a lot now, and I'm afraid that in the long run it will cost him his health."

In the same boat:
In the same situation (usually negative). Example: "I know exactly how you feel because I'm in the same boat."

In the works: Currently being worked on, developed. Example: "Our new website is in the works."

In and out:
Coming in and going out often. Example: "She's been in and out all day."

(To be) in with (someone):
(To be) friends or friendly with (someone). Example: "At high school, he was in with the wrong crowd. That's why he always got in trouble."

In advance:
Ahead of time. Example: "In the future, could you let me know about these things in advance?"