warren (n.)

late 14c., "piece of land enclosed for breeding beasts and fowls," from Anglo-French and Old North French warenne (Old French garenne) "game park, hunting reserve," possibly from Gaulish *varenna "enclosed area," related to *varros "post." More likely from the present participle of Old North French warir (Old French garir) "defend, keep," from Proto-Germanic *war- "to protect, guard," from PIE root *wer- (4) "to cover." Later especially "piece of land for breeding of rabbits" (c. 1400), which led to the transferred sense of "cluster of densely populated living spaces" (1640s).