One of the strangest and most dire tools utilized by the Scourge is the dreaded meat wagon. This rickety contraption is used to collect and store recently slain corpses from the field of battle. At any time, if the meat wagon carries a necromancer, ( corpses can be pulled from the meat wagon and raised into undead )warriors to swell the undead ranks. When upgraded, the meat wagon can also fling diseased corpses at enemy units with its crude catapult device. An invaluable support vehicle for the Scourge, the meat wagon strikes fear and woe into the hearts of even the most resolute defenders. ( Meat wagons use a Meat-A-Pult™ to fling corpses at the enemy. )
In World of Warcraft
This section concerns content exclusive to World of Warcraft.
In WoW, abandoned meat wagons can be seen littering the landscapes in various places on Azeroth, but are most prevalently found in the Western and Eastern Plaguelands, Eversong Woods, Tirisfal Glades, and the Ghostlands.
In Wrath of the Lich King
This section concerns content exclusive to Wrath of the Lich King.
A new type of meat wagon, seemingly made of Saronite like most newer Scourge objects, can be found throughout Northrend and around Acherus: The Ebon Hold. This model lacks a Meat-A-Pult™ entirely, and seems to be more destructive than its predecessor.
"Meat wagon" is slang for an ambulance.
See this subject on the Hearthstone wiki: Meat Wagon