Term used both figuratively and literally to disrespectfully slap someone in the face, with an open hand. It is often, but not necessarily, two or three quick slaps on a row.
Not unlike a "bitch slap" but used when even less regard is called for. Molly, in this instance, is referring to the term for a female cat, e.g., the way it playfully swats at things. The implication is that the person being slapped is not worthy of even a bitch slap, let alone a real punch.
When Jay tucked his thumbs in his fists to fight, Joey laughed, gave him a molly slap, and walked away.