All of this, and also, even if he WAS just playing with it, fucking around with a weapon is one of the ways that you get really good with it. With knives specifically, for a guy like Bucky — in both his lives — you’d pretty much have one on you at all times, and a lot of the military life (and probably the assassin life too) involves sitting around being bored as shit waiting for the death and terror to start. You end up playing with your weapons, because they’re there, and that’s one of the primary ways you really learn that weapon inside and out. You might play around, switching your grip, flipping it over and over, learning to catch it by the handle, by the point, learning to throw it, learning the exact weight and the center of its balance and all the other things that make handling it so effortless… it’s all just repetition and asking yourself “I wonder if I can….” and doing it until yes, indeed, you can stab some guy in the face before he can even see you coming.