This knife is one of Luger's default starting weapons. Note that I could NOT get the trophy to pop when using the melee button to stab - I needed to use the regular attack/fire button.
If you don't get it while playing through the game, just load up any level and pick Luger then go on a stabfest. I picked 3-1 since it's relatively straightforward, but any level should do. Note that even if you die, your progress seems to carry over.