'two-timer' originates from English, specifically from the phrase 'to two-time,' where 'two' refers to the number 2 and 'time' means to spend time with. The suffix '-er' denotes a person who does an action.

'two-timer' developed from the verb phrase 'to two-time' (to be romantically involved with two people at once), and the noun form 'two-timer' appeared in early 20th-century American English.

Initially, it meant 'a person who dates two people at the same time,' and this meaning has remained consistent in modern usage.