netori 寝取り

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In Japanese, netori 寝取り means "sleeping with someone else's partner," be it their girlfriend, boyfriend, wife, husband, or even someone who's not really their partner, like their childhood friend, or something like that.

Translation

Grammatically, netori 寝取り is the noun form of netoru 寝取る, which is a compound verb made out of neru 寝る, "to sleep," and toru 取る, "to take," so, literally: "to take [someone] by sleeping with [them]."

Manga: Seitokai Yakuindomo 生徒会役員共 (Volume 2, Page 138)

Context: Yokoshima 横島 tells a story about her seeing once again a guy that she used to like in high school.
demo kare wa sudeni kekkon shitete nee
でも彼はすでに結婚しててねー
But he was already married.
{netori ni kyoumi atta} watashi niwa zekkou no kikai!!
寝取りに興味あった私には絶好の機会！！
[It] [was] the perfect chance for [someone like] me [who] {had interest in sleeping with other women's husbands}.

This word is best known for its passive conjugation, ne-torareru 寝取られる, in noun form: ne-torare 寝取られ, which would be when your lover "is sleep-taken" by someone else, not when you sleep-take their lover.