Meaning of “maotoko(間男)”

“Maotoko(間男)” means “a man who is having an affair with married woman”.
I thought this is an internet slang, but in the fact this word exists from Edo era(1603-1868)! Wow!
But these days, it’s a slang I think.

“Ma(間)” means between, “Otoko(男)” means “man”.
So it’s easy to imagine the meaning of “maotoko(間男)”.

Imagine, maotoko is between husband and wife.
———————————-
[Husband] [Maokoto] [Wife] ———————————-

Example Conversation

A: Do you know she is cheating.
B: Oh, seriously?? Her husband is my co-worker!
He said he has a dilemma, maybe he realize about “maotoko(間男)”..

A: あの人不倫しているらしいよ。
B: え、まじで？？旦那さん同僚なんだけど！
　最近悩んでるって言ってた。間男の存在に気づいてるのかも…。