1842, "one who romps," agent noun from romp (v.). Rompers "small children's overalls" is attested by 1909, with ending perhaps on the model of trousers.
Entries linking to romper
romp v.
1709, "to play rudely and boisterously, sport, frolic," perhaps a variant of ramp (v.); but also see romp (n.). Meaning "to win (a contest) with great ease" is attested by 1888, in early use often in horse-racing. Related: Romped; romping.