Bumped your head 

You thought wrong

(Very common in the California Prison System, & on the streets)

John can you loan me some money

You bumped your head, no way am I going to loan you any money.

Bumped your head by Kconnor56 July 1, 2010

have you bumped your head? 

It essentially means you have you lost your mind? or it can be used to express your hatred or dislike for a particular activity or person, similar to kms.

"Moe's is better than Chipotle."
"Have you bumped your head?!?"
or

"This traffic makes me want to bump my head."

or
"Help! I have bumped my head and I can't get up!"

have you bumped your head? by colls42 December 4, 2013

Fell and bumped your head 

Meaning you must be seriously crazy. That you or someone else is experencing a really dumb moment at that time.

After Jimmy made the dumbest comment I ever heard I said fool you done "fell and bumped your head"

Fell and bumped your head by darrin2000 November 29, 2007