1. Telling someone to essentially give up whatever they are asking/demanding that you do because you will not do it.
2. More generally, telling an individual to "go fuck yourself."
1. Danny Glover (Murtaugh) in 'Lethal Weapon':
Bad guy, as he punches Murtaugh in the mouth: (Tell me what you know about) The shipment, Mr. Murtaugh!
Roger Murtaugh, blood and sweat dripping, tied up in a chair: Go spit!
2. Fuckwit boss: You need to rethink the way you're doing things on the job here because you're borderline on getting a contract extension.
Employee: You don't have the first fucking clue what you're talking about - go spit.