As a noun onesie is
(us) a one-piece garment for an infant or small child, generally worn over a diaper.
babygro
English
Alternative forms
* babygrow
Proper noun
(chiefly, British) An all-in-one suit for a baby made from a stretchy fabric; rompers.
onesie
English
Noun
(
en noun)
(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.
Synonyms
* (a one-piece baby garment) romper, romper suit