Zerging in WoW is a term that now means attacking an enemy (or a group of enemies) with a far bigger group of weaker units (or, alternatively, to keep resurrecting and running back to the fight, creating the illusion of an endless number of units). The word is derived from the Zergling rush strategy in StarCraft (also a race in Starcraft).
Normally, based on the Zergling Rush origins, the term application would be to sudden, speedy, surprise assaults to key elements (healers, flags, etc) by whatever sources are closest, likely few in number (at least initially) but has somehow reached a basic meaning of "big group winning" or "overpowering group" outside StarCraft, possibly from the initial attack group swarms in WSG openings.
For example, the original definition would parallel snatching a recently deserted flag in Arathi Basin, even by a solo player, while the current use would go along any big group formed to overpower a defended flag.
Blizzard, owning both the Warcraft and StarCraft names, has broken the fourth wall on some subtle occasions. In WoW, a zergling was a special edition non-combat pet, and an image of Infested Kerrigan is visible on Goblin Shredders.
In Warcraft III, two of the Zerg race units from StarCraft, the Zergling and the Hydralisk, were modeled in the game data. A Hydralisk was tucked into a corner of the map in one of the night elf missions, and both models appeared in a bonus ending to the credits after beating the Reign of Chaos on "Hard" difficulty. Editors could insert the creatures into custom maps through the World Editor.