This idiom or expression means:

Slow.
Not smart.

When someone says “He’s one taco short of the combination plate,” they mean “He’s not very smart.”

Also similar:
He’s not all together.
His upper floor is leaking.
He has some loose screws.
He hasn’t got all his buttons.
He has one ore out of the water.
The lights are out on the top floor.
There’s something missing up there.
The upstairs unit has been rented out.

This idiom or expression has to do with:

Keeping quiet.
Not saying a word.

I want you to button up about this meeting. No one is supposed to know about it, got it?!

This idiom or expression has to do with:

Making someone angry, usually on purpose.

A. Why can’t you two get along?
B. I don’t know! Somehow he always manages to push my buttons. I don’t like that.

Also, another usage:
Turning someone on sexually.

A. You like her, don’t you?
B. Yes I do. She knows how to push my buttons, and I like that!!

This idiom or expression means:

A touchy, sensitive, or controversial issue or subject.

Oil has become as much a hot-button political issue as health care.