Slapped patio head is referring to head that is being given on a patio. It is in reference to the gayest Smiths song ever 'Reel Around The Fountain'. If someone wants to slap you on a patio, they are probably wanting to give you the best head ever.
Sam: 15 minutes with you..
Hailey: Oh, it looks like she wants some slapped patio head!
Emma: I don't blame her...