Nipping is a common rabbit body language and one of the ways this pet sends some message to other rabbits, pets or even the owner.
A nip is a small sharp bite not intended to cause a lot of pain or break your skin when compared to rabbit bites which are more forcefully, will break the skin and may cause serious bleeding.
Why do rabbits nip?
There are many reasons why this pet might nip you. Therefore, to be able to understand the reasons, you need not overlook the circumstances under which this happened. Common reasons may include the following:
As a warning
Nipping is commonly used as a warning to other bunnies, pets or owners. They are trying to tell you they are not happy with what you are doing such as picking them wrongly (if they do not love being held) or politely telling you to get off their way.  Verbally, they are trying to say things such as ‘leave me alone’, ‘back-off’ or ‘you are on my way’.
Sometimes, it may be accompanied by other unhappy sounds such as snorting, hissing, whining, growling or a body language expressing anger or irritation such as biting, foot thumping, kicking, lunging, among others.
They are being territorial or want to establish dominance
Territorial bunnies, especially unneutered males or dominant ones may show this behavior to ward off any other rabbit, or pet or handler. It can happen on your hand or feet if you go too close or invade your bunny's territory. 
Also, it can happen when your bunny wants to take over some of the things to be his or hers such as a bowl, sofa, and so on. She or he may nip you deliberately for no apparent reason. It is a means to establish dominance and create a hierarchy. His or her expectation is you go and let him or her own whatever it is.
They need your attention
Sometimes, if rabbits feel ignored, they may nip you to get your attention. When seeking attention, they will often nip and run away.
They are afraid
If you startle your rabbit or do anything that scares it, it may nip you. Sudden rapid movements or grabbing him suddenly might cause this behavior. It is only a response to fear.
This is one of the courtship behaviors. During mounting, nipping may occur, and a small amount of fur may be pinched off on the back of the neck as the pet tries to grip its partner. This should not be interpreted as aggression.
Also, bonded bunnies, while playing together or chasing each other they nip one another occasionally. They are just playing, and this is only a game.
Finally, since they nibble and lick when grooming, it may be a normal sign of grooming other rabbits especially if they have been well bonded . The message here maybe ‘you are my friend now’.
Dealing with this behavior
You can squeal or screech if it happens, hold his or her head downwards for a few seconds to let your bunny know you are in charge or make him or her move out of the space by slowly tapping his hindquarters in case he or she is territorial.
Also, try the many other ways to deal with nipping and biting including spaying or neutering and training. With proper training, some may instead consider nudging your hand.