The story
From poopy + suit. A humorous acknowledgment of the difficulty of donning and doffing the garment, which is reputed to have resulted in self-soiling in wearers unable to remove one in time.
Etymology from Wiktionary, CC BY-SA 4.0.
poopy suit
Today it means: A waterproof overgarment worn by aircrew when flying over cold waters, which prevents hypothermia following ejection from the aircraft. Full definition →