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.
by Kconnor56 June 27, 2010
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"
by darrin2000 November 7, 2007
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?!?"

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

"Help! I have bumped my head and I can't get up!"
by colls42 December 5, 2013