In sex with multiple partner, 'hammerfisting' refers to the act of inserting ones fist into the vaginal- or anal orofice of a sex partner, whilst another sex partner repeatedly pounds the elbow of the inserted arm to create a hammer-effect.

to hit something or someone with a closed fist in an overhand motion intending to cause damage, similar to the way a hammer is swung using the tricep muscle. Usually quick and repetitive.

to plunge a fist repeatedly into a man's or woman's butt or vagina for maaaximum pleasure.