shirt [OE] The garments shirt and skirt [ME] share an ancient root, which is also that of short [OE], the basic sense probably being ‘short garment’. The idea behind shirty [M19th], ‘bad-tempered or annoyed’, is the same as that behind keep your shirt on [M19th], ‘don’t lose your temper, stay calm’. The offended or riled person is about to take his shirt off ready for a fight. In lose your shirt [M19th] or put your shirt on the shirt is seen as the very last possession that you could use to bet with.