It's a mission to acquire drugs.
Your friend picks you up bumping 2 Chainz and says "I just got to make one quick stop" and then you drive around aimlessly for approximately 24-72 hours, making multiple stops but never end up at the plugs. Then they drop you off at your house with no drugs and all you wanna do is sleep for 3 days.
Trever: "I just got to make one quick stop"
Josh: "dude I'm really not in the mood for a Fuckshit Mission"
Trever: "no it'll be quick I promise"
Delaney: " who could we call for a ride to the plugs house? "
Cody: "definitely not Maddi, she'll take us on a Fuckshit Mission"