Semen, specifically that of which is the result of fellatio. The use of the word is most effective around the holidays and in fall and winter. Neck Gravy is a bit too heavy and rich for hot weather.
"She gave me a blowjob and got a bigger helping of Neck Gravy than she expected."
"There's nothing wrong with that throat that a good dose of Neck Gravy won't cure."
"Pass the neck gravy, please."