Warren

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late Middle English: from an Anglo-Norman French and Old Northern French variant of Old French garenne ‘game park’, of Gaulish origin.

wiktionary

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From Middle English warenne, from Anglo-Norman and Old Northern French warenne (compare Old French guarenne, garenne(“game-park”)), probably ultimately from Frankish *warjan, from Proto-Germanic *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”). [1]

etymonline

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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).