Using somebody for their physcial skills and having no need for them to think or talk.
Listen, you're just a neck down, i don't need you to think, just do the work!
Someone who is useful, but only from the neck down; i.e. not someone to use for tasks that may require the ability to think or reason.
"Hey, Mike, can you get me a couple of neckdowns for that installation?"
A male or female that looks good as hell from the neck down...a.k.a. butter face
(everything looks good but her face)
He/she has a nice body, but an ugly face.
That guy has the coldest neck-down!