To roughen something is to make its surface less smooth — to give it bumps, scratches, or a grain. Skin can roughen in cold wind. A carpenter might roughen a piece of wood so glue or paint will hold. The word works two ways: you can roughen a thing, or a thing can roughen by itself (the road roughened after years of rain). It is a plain, slightly careful verb. In casual speech people often just say “make it rough” or “get rough.” Though actually, “get rough” can also mean people start behaving in a hard or violent way, so roughen is clearer when you mean a surface.