Etymology and Meaning of the phrases "Touched in the Head" and "Dropped on your head"

So I've been thinking about mental health/disability a lot lately, so I was wondering about the meaning of these phrases.

In 11th grade I read Grapes of Wrath and one of the characters calls another character "touched in the head". I assumed it meant mental disability (such as being kicked in the head causing brain damage) but apparently it means to be crazy/mentally disturbed according to dictionary.com and oxford. I say apparently because wikitionary uses mentally deficient and says a synonym would be the r-word and searching up the meaning lead to people who use it in that sense (including an MSW on quora). I don't know which is more correct

Second one, "dropped on your head as a kid" got me some flack. I argued that it implied mental disability from brain damage and I saw a movie clip of some little people telling a clearly mentally disabled clown that, to which he agrees. Others told me it just meant someone who is dumb. Even a r/disability mod. Is their an implication or am I just reading too much into it

https://redd.it/1wuuluh
@r_etymology