means A strip of material—most often a minimal piece of underwear with a thin back strap, but also a flip-flop sandal or a narrow leather lace—that gets its job done with as little surface area as possible.

from From Old English 'thwong,' meaning a thin strip or strap of leather, a cousin of words for binding and twisting. For centuries a thong was simply that: a leather cord for tying boots, whips, or sandals. The skimpy-underwear meaning is a 20th-century arrival, borrowing the old sense of 'narrow strip' and applying it where less is, apparently, more.