you must be smokin 

You must be trippin
You must be kidding me
You must be joking
You must be delusional

The boy said I can beat you in ball and the other boy says you must be smokin.
That mean the boy believes that he is delusional or dumb.
Like if your teacher says Philadelphia is a state and the students say she must be smokin.
"You must be smokin"

you must be smokin by GameGamer May 16, 2019

You must be smoking drugs 

Meaning that you are delusional

You must be smoking drugs if you agree with them.

You must be smoking drugs by FakeRiver June 29, 2021