two-time

 Longman Dictionary of Contemporary English 5++朗文当代英语 5++LDOCE 5++朗文 5++ˈtwo-time verb [transitive]  informalRELATIONSHIP to have a secret relationship with someone who is not your regular partner 背着…偷情；对〔恋人〕不忠 SYN cheat He doesn’t know Claire’s been two-timing him. 他不知道克莱尔一直对自己不忠。 —two-timer noun [countable]→ See Verb table

Examples from the Corpus

two-time• It turns out Ryan was two-timing Jeannie with her best friend.

ˈtwo-time verbChineseSyllable  

Corpus with to have a is who someone secret relationship

two-time
ˈtwo-time verb [transitive]
informal to have a secret relationship with someone who is not your regular partner
   SYN  cheat:
    He doesn’t know Claire’s been two-timing him.
—two-timer noun [countable]

two-timeBrE /ˈtuː taɪm/ 🔊NAmE /ˈtuː taɪm/ 🔊 verbpresent simple - I / you / we / they two-time BrE /ˈtuː taɪm/ 🔊 NAmE /ˈtuː taɪm/ 🔊present simple - he / she / it two-times BrE /ˈtuː taɪmz/ 🔊 NAmE /ˈtuː taɪmz/ 🔊past simple two-timed BrE /ˈtuː taɪmd/ 🔊 NAmE /ˈtuː taɪmd/ 🔊past participle two-timed BrE /ˈtuː taɪmd/ 🔊 NAmE /ˈtuː taɪmd/ 🔊 -ing form two-timing BrE /ˈtuː taɪmɪŋ/ 🔊 NAmE /ˈtuː taɪmɪŋ/ 🔊~ sb (informal) to not be faithful to a person you have a relationship with, especially a sexual one, by having a secret relationship with sb else at the same time 对（情人或爱人）不忠◆Are you sure he's not two-timing you? 你肯定他没有背着你另有所爱？🔊🔊▸ two-timer BrE /ˈtuː taɪmə(r)/ 🔊NAmE /ˈtuː taɪmər/ 🔊 noun