whale tail

noun

1. 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."”

2. A kind of large automotive rear spoiler.

   “the duck-tail spoiler was replaced with a whale tail for the US version of the Carrera.”

3. Used other than figuratively or idiomatically: The tail of a whale.