Whale Tail Meaning

(slang) An unintentional display of a thong or somesuch above the waistband of trousers etc.

Example: Used other than as an idiom: The tail of a whale.
An automotive spoiler
"the duck-tail spoiler was replaced with a whale tail for the US version of the Carrera." Porsche 911 Performance Handbook - Page 16. Bruce Anderson - 1997.