What is the origin of idiom forget your head if it screwed on?

The idiom "forget your head if it screwed on" is not a commonly used phrase in English. It's likely a misremembered or misheard variation of another idiom.

It's possible that you're thinking of one of these idioms:

* "Forget your head if it screws on" could be a mishearing of "Forget your head if it's screwed on", which is a more common, though still informal, expression. It means to be completely oblivious or unaware of something, often in a humorous context.

* It's also possible that you're thinking of "Forget about it", a common expression meaning to give up on something or not think about it any further.

* There's also "Lost your head", which means to have lost your composure or become irrational, usually in a stressful situation.

If you could provide more context about where you heard this phrase, it might be easier to determine the actual idiom you're looking for.