What is a goy?
A goy is a person who is a girl if examined at any time up to and including t, and a boy if examined at any time after t.
This comes from Ted Cohen's Jokes (Philosophical Thoughts on Joking Matters).
Cohen attributes it to George Boolos who attributes it to Richard Jeffrey who attributes it to Sidney Morganbesser if you're interested.
Apropos nothing, I am currently reading Boolos's book Logic, Logic and Logic (a gift from Rullsenberg) and I recalled this joke and thought I would contribute to the world stockpile of bad jokes.
Note to self: get out more.