Zac Efron -- That Awkward Moment ... Pretty Ballsy, Dude

Zac Efron is a brief guy ... at least he was on the set of his new film "Dirty Grandpa."

Efron is filming down in Georgia on Tybee Island and Monday he appeared to be baring it all. No such luck, just nude colored undies.

Funny, we had him pegged as a boxer guy but we're pretty sure there's a lot of ladies (and guys) who don't mind undies this tight.