Also called: Uruks, an anglicised plural form of Black Speech word Uruk: "Orc"
Uruk-hai is a Black Speech name meaning "Orc-race". These were a new kind of Orcs, bred by Sauron in the Third Age. They first appeared about 2475, when they conquered Ithilien and destroyed the city of Osgiliath. Soon also Saruman took up the breeding.
Contrary to the earlier Orcs, the Uruk-hai had straight legs, rose almost to man-height, and endured sunlight. Therefore they were chiefly used as soldiers by both Sauron and Saruman. The Uruk-hai thought themselves better than other Orcs, and called smaller kinds of Orcs Snaga, a Black Speech word meaning "Slave".
See also: Orcs
The Tolkien Encyclopedia
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