IDK, now that you mention it i do vaguely remember "noob combo" being thrown around, but i don't remember it being used all that commonly. Despite that nonsense on urban dictionary it wasn't exactly an insta-win combo, if you missed with the plasma shot then you'd lost, and it was pretty easy to dodge it at mid-long range. Honestly in BR games it wasn't really much of an advantage at all because at most ranges you could get off enough BR bursts during the plasma shot travel time to win anyway, as long as you were accurate, and at really close ranges it usually came down to who timed their melee the best.
That stuff on urban dictionary just sounds like sour grapes, it was the battle rifle that made the combo OP, the battle rifle was just broken in Halo 2. I suspect people just bitched about the plasma pistol because people were beating them with such a lowly weapon.
I guess what i mean though is it in comparison to the big special weapons, it was under-rated. Everyone always rushed for the sword or the rocket launcher or whatnot, when the plasma pistol could kill people just as quickly if used right. Though that's kinda why i loved halo so much, most weapons were very capable in the right hands/situation. Apart from the bloody BR, that game was well-balanced.