slap against (someone or something)
1. To make an audible slapping or smacking noise as a result of repeatedly striking or colliding with something or something. The tarpaulin came loose in the wind and started slapping against the side of the house. The dog's tail kept slapping against me while I ate my dinner.
2. To strike something against someone or something in such a way as to produce a slapping or smacking noise. In this usage, a noun or pronoun is used between "slap" and "against." She slapped the wet towel against my back. The wind keeps slapping the flag against the side of the building.
slap something against someone or something
to flap or strike something onto someone or something. The wind slapped the branch against Walter. The gusts from the storm slapped the shutters against the side of the house.
slap against someone or something
[for something] to flap or strike against someone or something. The flag kept slapping against Ed, making it hard for him to remain at attention. The awning slapped against the side of the house.