I was a kid in the 60's.. This phrase was not used by older sophisticated persons.. It was simply a juvenile response to juvenile name calling.. Some
dumb kid would say: "
You're Stupid" the simple and quick response would be "I know you are but what am I".. Sometimes (probably when first used) a good come back to one who was truly stupid, because it confounds them, confuses them.. "
Uh uh uh, um,
you're stupid?"
Example:
In actuallity, the phrase would go back and forth, several times.. "
You're stupid", "I know you are but what am I" "
You're dumb", "I know you are but what am I".. "What a jerk", "I know you are but what am I", "Jerk", "I know you are but what am I"...
Just a juvenile thing.. Used so often though (
way back when) that til this day, I use the phrase in response to good compliments.. Most people my age, or close too, enjoy and laugh :)
Example: "You're cute" my response "I know you are but what am I" :)