The story
From crinkle (“to rustle, as stiff cloth when moved”) + butt, because many disposable adult diapers have a plastic backing which makes such a sound. Compare crinkler.
Etymology from Wiktionary, CC BY-SA 4.0.
crinklebutt
crink·le·buttToday it means: A term of endearment for one who wears a diaper. Full definition →