Although it originated in the jail system as a term used when an inmate performed oral and also used as an insult to ones sexuality its no longer used in that way. Now it means to chill out
basically shut the F**K UP!!!
example#1:girl----- omg i can't believe you hooked up with another girl,how could you?!? guy--- rest your neck it aint that serious! example#2: guy-----man my swag on point nigg*s need to get on my level! guy#2-----bruh seriously??? rest ya neck, you 30 and still live at home with your mother! example#3: guy----lakers are the G.O.A.T! guy#2----pshh...rest your neck!!! Celtics all day!!!!