What do you say when a person/an idea/… is crazy to your mind?

Question

What does one say or do if they think that a person's idea, behaviour, etc. is crazy to their mind?

For instance:
In Germany, when someone dangerously passes you, you will show them by tipping your finger on your forehead(or temple) that they "has a bird" _{(literal translation of "einen Vogel haben")}. This action is called "jemandem den Vogel zeigen", literally translated: "to show someone the bird".

I found the phrase "bats in the belfry" which has a similar meaning to the German phrase. I haven't heard it before, and I'm not sure if this phrase is in common use.

What do you usually say? Do you also have a gesture like tipping your finger on your forehead/temple? And is it insulting by law?

---

I'm interested in both American and British phrases and gesture.

Just as side note: In Germany it is highly controversial if it is an insult by law or not. There were several court decisions pro and contra.

Answer 1

You're asking about three things: a gesture, an idiom, and showing someone the bird.

One gesture sometimes used in the U.S. to indicate "you're crazy" is pointing a finger at your temple, and turning it in a small circle a few times. I found one blogger who wrote, while this is rather benign in the U.S., it's regarded as highly offensive in some parts of Europe.

We express the meaning of that gesture by using terms like "you're crazy," "you're nuts," or even "you're loco," borrowing from the Spanish. "You're cuckoo" is also well-known (I guess a lot of cultures associate birds of some sort with a loss of sanity or judgement). The idiom "bats in the belfry" is well-understood, but not all that commonly heard; a few more popular idioms include, "has lost a few marbles," and "has a few loose screws;" I think the latter might be more likely to be applied to a single rash act (such as passing recklessly on the highway).

In the U.S., to "flip the bird" is to point your middle finger at someone. You might do this when they pass you unsafely on a roadway, but it hardly means "you're crazy!" (actually, it means "F*** you!"). That gesture is considered vulgar and offensive; I'm don't know if someone would or could be prosecuted on the roadway, but it evidently does violate obscenity laws when broadcast on television.

Answer 2

The corresponding gesture in the English-speaking regions is pointing a finger at the temple while turning the closed palm: Screw loose!

It suggests that a screw (a threaded nail) seems to have come loose in the person's head and tells him to tighten it thus: by turning it clockwise!