Definition from Wiktionary, the free dictionary
From the Gerber Childrenswear brand name Onesies. The name became generic and was reinterpreted as a plural, hence the new singular and decapitalized "onesie".
onesie (plural onesies)
- (US) A one-piece garment for an infant or small child, generally worn over a diaper.
- One-piece adult loungewear jumpsuit.
- One-piece fashionable streetwear worn mostly by teenagers.