fisting

s, g, nyrkkinainti (present participle of)
puhekieltä The sexual practice of inserting one or both hands into the vagina or rectum of one's sexual partner.
2003 August 17, Sean O'Hagan, “Sex on the brain”, review in The Observer, page 5,

(..) and comes to a climax of sorts (sorry) with – and here the more sheltered among you should avert your eyes – a girl-on-girl fisting scene.