D-neck refers to a trendy shirt, often worn by d-bags, with v-neck shaped collar that dips so low you can see the person's chest.
"Baby, it's not a v-neck, it's a d-neck. My collar goes so low you can see my chest" -The D-Neck Song
A D-neck (Dirted Neck) is when the collar of your shirt is stretched/reemed the hell out. to the point where like you can see some of your chest. people tend to act cool. half the time they are d-necking. yes it is a verb too.
dude..tim thinks he's cool..but little does he know hes d-neckin hard as shit