If you smack something somewhere, you put it or throw it there so that it makes a loud, sharp noise. He smacked his hands down on his knees Ray Houghton smacked the ball against a post
If you smack someone, you hit them with your hand. She smacked me on the side of the head. Smack is also a noun. Sometimes he just doesn't listen and I end up shouting at him or giving him a smack