While your request for likely refers to the classic soul ballad by Billy Paul , there are a couple of other popular references this could mean. Here are the most likely interpretations:

Written by Kenny Gamble, Leon Huff, and Cary Gilbert, the song was inspired by a real-life observation. The writers noticed a man and a woman who would meet at a bar in Philadelphia every day at the same time, sit in the same booth, and then leave separately. While the writers didn't actually know the pair, they imagined a secret, forbidden romance between them and turned it into a chart-topping hit.

Since this is the most direct match, The Story Behind the Song

: The singer acknowledges that they "both know that it's wrong," but the connection is "much too strong to let it go now".