verb phrase
(Variations: by or down or out may replace up) To come for a visit; arrive: The bash is in the basement, Dad. Fall up anytime (1930s+ Black)
Prithee did he fall up the steps or down the steps, Uncle Dick?
That is, it will now fall up as rapidly as it would previously have fallen down.
That was just the moment to "fall up against" the hard-surfaced man.