Onesie/ˈwʌnzi/ Definition 1 of 3Pro Tip 1/3Infant Comfort Onesie provides comfort and convenience for babies, as it's easy to put on and take off. Her newborn seemed cozy in the striped onesie. Pro Tip 2/3Adult Casual Adults wear onesies for relaxation, often at home or during informal events. They threw a onesie-themed party for her birthday. Pro Tip 3/3Not for Work A onesie is too casual for professional settings and shouldn't be worn to work. He quickly changed out of his onesie before his job interview.