'strappy' originates from English, formed by adding the adjectival suffix '-y' to the noun 'strap', where 'strap' meant a strip, thong, or loop used for fastening.

'strap' comes from Old Norse 'stropp' (or Old English/Medieval forms like 'straep'), meaning 'loop' or 'thong'; in modern English the noun 'strap' continued to mean a band or strip used for fastening, and the adjective 'strappy' developed by suffixation in recent centuries.

Initially related to the physical object 'strap' (a band or thong), over time the derived adjective came to mean 'having straps' and extended in informal use to imply a strap-revealing (sometimes sexy) style.