booty flag
A less severe version of a "whale tail". When the triangular portion of the thong and the waistband peeks out above pants, skirts or shorts.

Pennant booty flag. When both thong waist band sides are visible along with the triangle forming a pennant flag-like display.

Booty flag is also useful when you want to tell your girl she has a whale tail but you have been told not to on account that she does not want to be described as anything relating to a whale.

Also useful in the probable event that a woman's thong shows at a pirate/nautical themed party.
Pirate: Arrgggh, avast me hearty, ye be showin' yer whale tail!
First Mate: There's a whale out there?!
Pirate: Argggh, no, ya scallawag, your thong is showing!

Guy: Hey, babe?
Girl: What?
Guy: I think your whale tail-I mean, your booty flag-is showing.
Girl: Oh crap I hate when it rides up on me like that.
Guy: I don't.