I don't know
What does idk stand for

"idk" stands for "I don't know", but I'm sure you're just after the old joke that goes along the lines as follows:
Person A: "What does 'idk' stand for?"
Person B: "I don't know."
Person A: "I don't either, that is why I'm asking..."
and so on...
Nice try @rychessmaster1 but you can do better than this

Mom: Child, what does IDK, LY, TTYL stand for?
Child: I don't know, Love you, Talk to you later.
Mom: Ok, I will ask somebody else who knows.

I Do Know
You do?
Yes, yes I do.
Then what is it?
You wish you knew

Mom: Child, what does IDK, LY, TTYL stand for?
Child: I don't know, Love you, Talk to you later.
Mom: Ok, I will ask somebody else who knows.
LMAO

Mom: Child, what does IDK, LY, TTYL stand for?
Child: I don't know, Love you, Talk to you later.
Mom: Ok, I will ask somebody else who knows.
The child doesn’t know either? UGH
I think you are joking, but if you are not, then the child is saying the expanded form of the three. Get it?
Anyone?