As a proper noun mommy
is (chiefly|used by young children) one's mother.
As a noun onesie is
(us) a one-piece garment for an infant or small child, generally worn over a diaper.
mommy
English
Noun
(mommies)
(US, Canada, usually, childish) Mother.
Synonyms
* ma, mama, mamma, mom (qualifier), momma (US), mum (British), mummy (British)
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