The term underconned (or undercon for short) implies that a monster is exceptionally strong for its level. (That is, it is "underconsidered" by the in-game consider system.) This could imply, for example, that a heroic mob has hit points on par with an epic encounter.
Some notable underconned monsters include Spirit of Do'ellin, a 70 ^^^ heroic dragon for the Draco Mortuus vos Liberatio heritage quest and young dragon, a 65^^^ heroic dragon for the Wondrous Inventions of a Crazed Gnome heritage quest.
The term is to be used in Wiki with care. A kill that is simply hard or requires some strategy is not an underconned monster unless it requires an exceptionally long time to kill (measured in several minutes) in a strong full group of at least the same level, does exceptional AoE damage and/or has exceptional hit points.
Naturally, heroic or epic monsters are not underconned even while they are harder to kill than average or normal monsters. Heroic or epic monsters are shown in-game as requiring a group or a raid party, and (usually) give rewards on par with their difficulty.