Best definition
A building without elevator.

When you live in a walk-up the worst thing that can happen to you is to break a leg.
Walk-up: define #2
walk up
To seduce someone while walking up to a person; place or thing. Commonly used in a sentence casually.
Billy texted Jodie, “hey wanna walk up to the resaurant?”

While he was thinking, “I’m gonna get some.” she had pepper spray in her purse.
Walk-up: define #3
A slang word for a “dirty” policeman,or a policeman whose simply nothing short of a DICK
I was minding my own business walking down the street an this fucking Walkup took me to jail.