Slang mainly used in North America, refers to anal sex, using the fist. To attempt to place one's entire hand, or both hands, into another's anus, and then balled up into a fist, though not always.
Not fully considered SM due to its need for both parties to provide assistance, in accomplishing the act.
Additionally, it can be a sexual activity on its own, where there is no dominant partner, meaning that both are seeking the satisfaction of being both the giver, and the receiver.
Fisting, while conjures the idea of a clenched hand being inserted into the rectum, the reality is that this usually is only done, once the hand is fully inserted.
Further, it does not always occur where all fingers of the hand, can be inserted, though still referred to as being fist fucked.
Proper lubricants, and nail care are essential. Long nails by the one doing the fisting is a major injury risk.