When you do something that’s going to end in certain failure but you decide to do it because it’s your obligation to do so. It’s also a very British thing to do.
Can be used in a video game context when you sacrifice yourself for the better of the team.
Variant : suicide mission
Dude1: Cya dude im going on a suicide mission and gna call my ex up about going out again.
Dude2: Instant death.
Dude1: No he’s got a rocket launcher.
Dude2: Don’t worry I'll go outside on a suicide mish and waste his ammo.
Dude1: Dude I got this girl into my room and I was so horny i asked her to suck me off, I thought i was on a suicide mission, but she unzipped my pants and sucked it like a pro whore.
Dude2: Lucky fuck, can I have her number?