Smack has several meanings:

1. To hit something with a quick, sharp blow, usually making a loud sound. You might accidentally smack your knee on a table corner, or smack a mosquito that's biting your arm. The word captures both the action and the sound: when you smack your hand on a desk, everyone hears that sharp crack.
2. To make a loud noise with your lips, often when eating or tasting something. If you smack your lips while chewing gum, you're making that popping, clicking sound that can annoy people around you.
3. To place or move something with force and noise. A judge might smack down the gavel in court. You might drop your backpack and hear it smack against the floor.

The word sometimes appears as smack-dab, meaning exactly or precisely in a location: “The library is smack-dab in the middle of town.” As an adverb, smack can mean directly or exactly: “She walked smack into the glass door.”

When someone does something smack in front of others, they're doing it openly and obviously, right where everyone can see it.