(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.

plural of onesie

A call in any childrens game, such as jacks, where one action must be performed.

Wearing a onesie.

(idiomatic) Individually or in very small groups, rather in larger batches.