Onesie comes from the word "one" and means a one-piece suit, usually in the form you use for lazy days at home. It is also used for one-piece snowsuits and one-piece motorcycle suits. The word is sometimes spelled onesy
The name may be derived from the registered trademark Onesies
®, which is a range of clothes for infant children. The word onesie (in singular) does however, in modern usage, more often mean a suit in adult size.
When I come home from work, I just like to change into my onesie and watch TV.