To disrespectfully slap someone in the face with an open hand. The implication is that the person being slapped is not manly enough to be worthy of a real punch, Different from a "pimp slap", which uses the back of the hand and is usually disciplinary.
The kind of slap a pimp gives to his whores to keep them in line or punish them. However, it is most commonly used to describe an insulting slap from one man to another, as if the slapper is treating the slappee as his bitch.