cheating on hisoka
"cheating on hisoka" means that you are in a relationship with a man named hisoka, but you love genocide jack more than him so you simply cheat on hisoka.
killua: "did you see that girl komaru? she was with that girl genocide jack."
tsumugi: "yea... she was cheating on hisoka.'
tsumugi: "yea... she was cheating on hisoka.'