She found out her boyfriend was a two-timer, dating two girls at the same time.

two-timer 是由 two（二）和 timer（计时者）组成的复合词。timer 来自动词 time（计时），加上后缀 -er 表示"做某事的人"。 two-timer 字面意思是"做两件事的人"，引申为"同时与两个人约会的人"，即"脚踏两条船的人"或"感情骗子"。