From Middle English warenne, from Anglo-Norman and Old Northern French warenne (compare Old French guarenne, garenne (“game-park”)), probably ultimately from *warjaną (“ward off, defend against”) (compare also Old French warir, guarir, a borrowing from this Germanic root). Alternatively from Gaulish *warrennā (“enclosed area”), from *warros (“stick, post”), Proto-Celtic *warrā (“post, prop”). 
warren (plural warrens)
- The system of burrows where rabbits live.
- An enclosed piece of land set aside for breeding game, especially rabbits.
- (figuratively) A mazelike place of dark alleys etc in which it's easy to lose oneself; especially one that may be overcrowded.
- A heart-shaped hoe.
- Plural form of war