Green’s Dictionary of Slang

kicks n.1

[ety. unknown; ? link to UK dial. kecks, the (dried) hollowed-out stem of an umbelliferous plant, e.g. a teazle; such stalks were used as candlesticks, water-pipes etc, and the link to trousers, themselves ‘hollow stalks’, seems feasible]

[late 17C–1930s] (also kicksters) breeches, thus trousers.