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1.
Being a mistress, or just a girlfriend on the side.
"My God, after sexing Mandy, Jake went home to his wife; Guess she's his new sideline ho."
by Aidenne February 20, 2007
 
2.
The act of a team member being on the side line and talking smack or fooling around.
Joe (wide receiver): Hey, What I say, Sideline Ho!
by BrainsForBrains September 04, 2009