劈腿 (pi1 tui3, verb), literally means to split legs, is commonly used to describe one's cheating on his or her partner in a relationship. Similar words in Chinese are:
外遇 wai4 yu4 (n) an adulterous affair
出軌 chu1 gui3 (v) to leave the rails; to overstep bounds; to cheat
偷吃 tou1 chi1 (v) to eat sneakily
