2.) To run around on' a boyfriend/girlfriend, or wife or husband. To see another for romantic or sexual reasons.
3.) To give the run around, to tell someone something not entirely true to lead them to someone else to ask, than to someone else, so the person never gets the right answer.
2.) "Hugh is running around on Cindy again isn't he?" - "Yeah, I saw him with some 21-year old cocktail waitress at the bar last night, he's really giving her the run around."
3.) "Where do babies come from?" Asks little Emily. "Uh, the sky, I don't know go ask your mother." - "Mom, where do..?" - "I don't know, go ask your grandma." - "I don't know, go ask your grandpa," - "I don't know, go ask your Dad." And so on, and so on.