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?"
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" :)
by iBilly October 1, 2013
The best come back to any insult in the history of the planet. Using this phrase will completely destroy and annihilate your component, so only use of absolutely necessary. (Say they think Sam and cat is a good TV show)
Fred:Our mom just died

Joe:I know you are but what am I
by •*MentallyMaggie*• March 9, 2020
Basically you can't come back from it unless you use an Uno reverse card. An op roast for kids 7-10
Gay1:your mum gay lol
Gay2:no u

Gay1:uno reverse card

Gay2:I know you are but what am I

by Oh crackers April 26, 2019
The gen z way of saying haha bitch I won the argument

It is literally the worst comeback of all mankind
You are dumb

I know you are but what am I
by 42069ultimateholdersex April 2, 2021
A great comeback or a phrase that will wreak havoc. This phrase and be used to simply annoy someone or to say for the heck of it.Also it can be used in a conversation.
1)Ruben:Your a butt cheek

Tom:I know you are but what am I?

2)BOB:I'm going to be Albert Einstein

Rick:I know you are but what am I?
BOB:Beats me it's up to you
It's quite simply the most retarded comeback someone makes when they've completely lost the argument. This statement signifies that the argument has gone to a completely immature level. Below is a hardcore argument starting with the immature statement :)
Person 1: I know you are, but what am I?

Person 2: Good question. What are you?

Person 1: Wow, you're so dumb, you don't even know what I am?

Person 2: You're so dumb, you can't even answer my question.

Person 1: I don't answer stupid questions. Sorry.

Person 2: Then you shouldn't have made your first one.
by Searingwolfe July 13, 2011
the most offensive comeback to ever be said. this comeback makes "your mom" seem like a nice thing to say. this comeback can out roast any insult thrown your way.
person one:You gay

person two: i know you are, but what am i

whole everyone else: OOOOHHHHHHH
by BlueStripes March 19, 2019