in reference to navels, by 1972, from in (adj.) + -ie.
Entries linking to innie
in adj.
"that is within, internal," 1590s, from in (adv.). Sense of "holding power" (the in party) first recorded c. 1600; that of "exclusive" (the in-crowd, an in-joke) is from 1907 (in-group); that of "stylish, fashionable" (the in thing) is from 1960.
-ie
alternative spelling of -y; now mostly of -y (3), but formerly of the others as well.