No scoping is generally viewed as an act of skill (or luck, depending on who you talk to and the mechanics of the game you're playing). This is because you are killing someone with one shot through a (usually) large crosshair and also risk exposing yourself if you miss. However, the amount of prestige that is attached to a single no scope depends on the distance and rate of success; i.e. if you no scope once and do this by killing a guy who you are literally next to, that doesn't take much skill, but if you continually no scope guys who are halfway across the map, that's some seriously good shit on your behalf. On a related note, no scope headshots get you some serious prestige if you can pull them off constantly.
Player 1: Damn, I just no scoped some guy on
Commonly used in Halo with a sniper but not limited to.
No scoping is NOT applicable to weapons where scoping is not required. ex. ak47, m4a1, battle rifle, SMG, etc.
Blue guy 2: Nice!
Dead guy: I just got owned..
Everyone: OOOOH noscope!!!!!!!