whale tail

noun2

1. •
   (slang) An unintentional display of a thong above the waistband of trousers etc.

   • So, next time you see a mom with a "whale tail" peeking out of her pants while she puts in a car seat, remember, "You can't judge a book by its cover." — 2007
2. •
   (slang) A kind of large automotive rear spoiler.

   • the duck-tail spoiler was replaced with a whale tail for the US version of the Carrera. — 1997