Verb: To do something that is incredibly awesome, rad, or bad ass
Buster: Guess whathappenedyesterday?
Chaston: What?
Buster: This guy tried to mug me, but I stabbed him in the neck with my dagger.
Chaston: Whoa! That's dagger!
Buster: I know. I totally daggered him.
A reply to an insult, usually exclaimed by a third party witness to one person insulting another, praising something particularly crafty or well thought out. Often referencing the use of an inside joke, or something of a private nature between the people conversing.
Person (A) says a witty "Your Mom" joke, insulting Person (B)'s mother. Person (C) exclaims; Damn dude, that's a dagger!
in basketball when a player is on a fast break with no one in front of him but pulls up behind the 3 point line and shoots it, yelling "DAGGER!" as they shoot. (it only works if you make it)