Ah, the lingo from the good old days. As MikeBrooklyn pointed out, we used the word "peeker" to describe a belly button that's just "peeking out" from under the waistband. Since modern fashion trends have paired crop tops with high-rise bottoms, "peekers" are quite common these days. We also had another term to describe the inverse of a "peeker" — when a girl was wearing low-rise bottoms that exposed the lower portion of her tummy, but with a top that *didn't* fully reveal her belly button, we called it "foofing". "Foof" never quite caught on like "peeker" did, and now that low-rise bottoms are no longer en vogue, you don't see "foofs" very often.
