onesie/ˈwʌn.zi/EtymologyFrom the Gerber Childrenswear brand name Onesies. The name became generic and was reinterpreted as a plural, hence the new singular and decapitalized onesie. Equivalent to one + -sie.onesie means Any of various one-piece attires Lexicurio rates it Sui generis — a strength score of 88 out of 100.nounAny of various one-piece attires“It was once considered taboo to even suggest that new motherhood was not all sunshine onesies and rainbow mobiles, and “Baby Ruby” arrives on a welcome wave of contemporary movies exploring how the joys of child rearing can commingle with misery.”Any of various one-piece attires:; A one-piece garment for an infant or small child, generally worn over a diaper.Any of various one-piece attires:; A one-piece adult loungewear jumpsuit.Any of various one-piece attires:; A one-piece fashionable streetwear worn mostly by teenagers.