An Orc Bully commanding a crew of Goblins
Rock Lobbers are a crude variant of the catapult or trebuchet which was first replicated by the Greenskins when they encountered Dwarf siege equipment during the onset of the Goblin Wars. Most Greenskins, especially Orcs, were never mindful about such complicated contraptions, but they do have a great appreciation for weapons that prove destructive upon the battlefield. This appreciation eventually led to innovation, and in time the first Rock Lobber was used extensively against the Dwarfs for many centuries to come.
Since then, the Greenskins have built a plethora of what they generally refer to as "Lobbers". These artillery pieces are often deployed either singularly or in batteries that helps to either soften up enemy formations, smash defensive positions, or as a means for a Greenskin warrior to fling himself across the enemy battlements and directly into the city streets. Rock Lobbers come in all sorts of shapes and sizes, but the most common variant is a counterweight device similar to a Trebuchet. Due to its complicated design, most Rock Lobber are usually crewed by Goblins, often under the command of an Orc Overseer, also known as Bullies. Goblins are often very lazy in their duties, so it is often necessary for their Orc overseers to whip these Goblins into action, cranking up gears and pulling on ropes to ensure that the artillery continues its steady stream of barrage.
Source
- Warhammer Armies: Orcs and Goblins (8th Edition) -- pg. 47