Neck is a slang term used for fellatios, head, oral sex etc. For instance if someone is asking for a blowjob, they will instead say 'Give me some neck'. This slang is mostly used to refer to a woman giving oral sex to a man, however, it can also be used for a man giving oral sex to a woman. This slang is heavily used in hip hop songs, in L'A Capone's 'Facedown' song, he said "She give me her check, she give me her neck", L'A is basically saying not only does her girl give him money, but she also give him oral sex.