Green’s Dictionary of Slang

a protruding navel, as opposed to an innie n.

1972		Wash. Post 22 Oct. (Potomac section) 19/1: Any number of permutations can be wrought with that useless ornament, the belly-button; it can be raised, lowered, enlarged, reduced, made to obtrude (an ‘outie’) or to recede (an ‘innie’) [OED].
1973		Tharunka (Sydney) 8 Nov. 28/1: Is he coming? Is he what. This sat sang is only for starters, honey. Step up to a fish-eye-glazed outie.
2002		Guardian 17 Feb. 🌐 A diamond stud twinkles in Britney’s navel [...]. Her tummy bud is an innie, not an outie, so the incision was excruciating.
2004		Mad mag. Feb. 34: Very sensitive to jokes about her ‘outie’ belly button.
2022		Twitter 16 Mar. 🌐 Is that an outie belly button.