1.) Slang for blowjob, or oral sex performed by females.
2.) A way of someone receiving pain for stupid thoughts or ideas by giving them a hard slap on the back of their neck.
3.) The area between your head and your shoulders.
to make out, among other activities, with another person. A rather archaic term for it (used particularly well in The Catcher in the Rye, for instance), sometimes used in the present day.
"Hey , did you hear that Applebee's is no longer going to sell Long Island Iced Teas?"
"Yeah, I'm going to neck myself. Guess we'll have to drink somewhere else"