A gentle tap on the body of another, as a sign of praise or affection. Whilst it's usually done with a fist, it's not meant to cause any pain.
Think of it as a "fist bump" but instead of two people complimenting each other, it's "one-sided". So instead of two fists meeting, it's from one person's fist onto the body of another person, usually the shoulder, chest, or another part of the torso.