The Manhattan Back Smack is when your conducting anal sex with a gagged hog tied lady while under the effect of at LEAST 10 Viagra pills, you then smack her in the back of the head with your penis until she becomes disoriented OR passes out.
You finally finish the job by unloading a sea of hot sticky man milk on to her face...Then run like hell
Jerome: Bro! Where did you disappear last night?
You: Hey you know that hot piece of ass from last Friday?
Jerome: yeah what about her?
You: Well guess who gave that bitch the Manhattan Back Smack!