lair (n.)

Old English leger "act or place of lying down; bed, couch; illness; the grave," from Proto-Germanic *legraz (source also of Old Norse legr "the grave," also "nuptials" (both "a lying down"); Old Frisian leger "situation," Old Saxon legar "bed," Middle Dutch legher "act or place of lying down," Dutch leger "bed, camp," Old High German legar "bed, a lying down," German Lager "bed, lair, camp, storehouse," Gothic ligrs "place of lying"), from PIE root *legh- "to lie down, lay." Meaning "animal's den" is from early 15c. Essentially the same word as layer (n.), but more ancient and differentiated in sense.