Two-timer noun
Used to refer to a person who cheats his/her partner by having a secret sexual relationship with someone else.
Although his wife loves him deeply and supports him wholeheartedly, he is still a two-timer. I saw him going on a date with a strange girl last weekend.
Used to refer to a person who says in one way but acts in another way.
We knew Jack was a two-timer, so we didn't tell him about our real plans.
You shouldn't believe in her words. She's a two-timer. I used to be cheated by her.
The verb form of this noun, "two-time" means "to deceive, cheat, betray". This meaning dates from 1924, perhaps from the notion of "to have two at a time". An earlier reference (1922) in a Kentucky criminal case involves a "double-cross" or "betrayal" without a romance angle.