Green’s Dictionary of Slang

an indented navel; a person with such a navel.

1966		J.D. Ball et al. Loosely Speaking: Centennial Lex. Carleton Neologisms (Carleton Coll.) 11: Innie, a human being with a concave navel (i.e. most of us) [OED].
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].
1997		C. Newland Scholar 93: A short navy blue top that ended just below her breast, exposing a tiny belly button. Cory smiled to himself as he noticed she was an insy.
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.