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1.
A trademark registered to Gerber Childrenswear LLC, often used in connection with a one-piece garment for an infant or small child, generally worn over a diaper.
The child wore a Gerber Onesies garment.
by Onesies March 26, 2009
 
2.
A selfie taken by HM the Queen
One has taken a onesie on one's mobile
by cousinjack49 March 16, 2014
 
3.
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.
by EliasG June 04, 2013
 
4.
When a girl is so tight, you can only fit one finger in her.
Emma's a Onesie!
by Lateofthepier October 20, 2009
 
5.
a single cigarette sold over the counter by a retail establishment, instead of a pack of 20. useful in New York if you don't have the $8 for a pack. also useful for the quitting smoker who just needs one nicotine hit to take the edge off. onesie's are illegal in some states - eg Illinois 2009
"he sold me a onesie with a cup of coffee"
by mak108 August 22, 2009
 
6.
to masturbate, have sex with oneself, to commit onanism
...and I caught that bitch having a onesie in the middle of the tennis court.
by BabyA December 09, 2005
 
7.
A self rolled spliff that is the size of a rollie cigarette, using one regular sized rolling paper/skin.
Dave: Is there any weed left?
John: Nah man, not even enough for a onesie.
Dave: :(
by Kempernol January 08, 2008