To hit something with a loud noise, like a big slap or a punch.
Swack can make people think of fighting or of something hitting something else very hard.
Examples:
He swacked the table with his hand.
She swacked the ball with the bat.
The door swacked against the wall.
The cat swacked the mouse and it flew away.
The giant swacked the tiny man and he flew into the sky.
The tree swacked the car and it broke.