smack 1 
(smăk)
(smăk)v. smacked, smack·ing, smacks
v.tr.
1. To press together and open (the lips) quickly and noisily, as in eating or tasting.
2. To kiss noisily.
3. To strike sharply and with a loud noise.
v.intr.
1. To make or give a smack.
2. To collide sharply and noisily: The ball smacked against the side of the house.
n.
1. The loud sharp sound of smacking.
2. A noisy kiss.
3. A sharp blow or slap.
adv.
1. With a smack: fell smack on her head.
2. Directly: "We were smack in the middle of another controversy about a public man's personal life" (Ellen Goodman).
[Perhaps of Middle Flemish origin, or perhaps of imitative origin.]