I .
( informal ) to increase the price of sth suddenly :
They've slapped 50p on the price of a pack of cigarettes.
II .
slap sth on sth | ˌslap sth ↔ ˈon
to spread sth on a surface in a quick, careless way :
Just slap some paint on the walls and it'll look fine.
I'd better slap some make-up on before I go out.