well it's just what it sounds like. Having your neck licked by some groupie or whatever without your total consent in Shanghai or anywhere.
"Standing at the rim of the violence, I was groped by a drunk punker, complete with a shanghai neck-lick. My contempt for all-things-androgen reached its peak, and, smearing the saliva off my collar, I mined my purse for keys."
Someone Shanghai neck-licks and slobers on your neck or collar.