means A one-piece garment, especially a loose all-in-one outfit for lounging or sleeping, modeled on an infant's bodysuit but sized for adults and older children.

from From 'one' plus the diminutive-playful suffix '-sie' (as in 'oopsie' or 'footsie'), originally trademarked baby talk for a one-piece infant bodysuit in the mid-20th century. The adult lounging version — and the word's leap into general use — is a much more recent development, riding a wave of comfort-clothing popularity in the early 2010s.