Side-Slipper 

A person who will initiate and wishes to, or attempts, to become involved, usually romantically, with a man or woman, who is already in an established relationship, (e.g., married, engaged, boyfriend/girlfriend, etc.), but will not put forth the effort to obtain and establish a relationship of their own. To engage in an affair or come between two parties. Such relationships might be deemed genuine, at least from the perspective of the "side slipper," or merely an attempt by such a person to interfere with the sole purpose of causing temporary or permanent disruption of the established party's relationship. (May also be seen or written as "Side Slipper").

Variation: To "side-slip."

Josie just can't seem to get away from the life of being a side-slipper. She keeps trying to get with Curtis but knows he and Laura are engaged to be married. She claims she is actually in love with him, but I think she merely wants to cause trouble to break them up. If she spent more time trying to find someone of her own, she wouldn't have time to worry about what others are doing.

Larry thinks he can slide-slip between Diane and Ronnie. He's been after her for a while, but might experience a rude awakening.

Side-Slipper by Who 'Dis? Who 'Dat? February 13, 2019

Side Swipper 

Someone who has a strikingly sexy profile (side view... not online), but they are not as hot from the front.

Ey, remember that dude i was scoping at the Kwanzaa Festival sitting cross the room- I saw him from the front and he was a straight Side Swipper, dis-a-ppointment!

Side Swipper by Chellz January 5, 2008

Cleveland Sideslapper 

When the female vagina has become loose because of overusage the sides of the vagina flaps as she walks. Science has shown that all girls in cleveland have loose pussies.

Her cleveland sideslapper hit me in the face and gave me a black eye.

Cleveland Sideslapper by Ramchargerz April 27, 2009