Expression used to show that someone would agree to some sort of condition if highly pressured. It's usable in place of a "no."
1: Would you sleep with Dave Grohl?
2: I'm a dude and I'm not gay!
1: I know you're a dude, but come on!
2: well, gun to my head, I guess he's not too ugly.
usually used when someone is asking a tricky ass question or proving someones knowledge of some shit
person 1: yo bump that new dmx
person 2: since when did you fuck with dmx?
person 1: since day 1 bro trust (he capping)
person 2: word? gun to your head name me 5 dmx songs