Is it ensure, insure, or assure?
Old English denn "wild animal's lair," from Proto-Germanic *danjan (cf. Middle Low German denne "lowland, wooded vale, den," Old English denu "valley," Old Frisian dene "down," Old High German tenni, German tenne "threshing floor," from PIE *dan- "low ground"). Sense of "small room" is 1771, originally colloquial.
a lair of wild beasts (Ps. 10:9; 104:22; Job 37:8); the hole of a venomous reptile (Isa. 11:8); a recess for secrecy "in dens and caves of the earth" (Heb. 11:38); a resort of thieves (Matt. 21:13; Mark 11:17). Daniel was cast into "the den of lions" (Dan. 6:16, 17). Some recent discoveries among the ruins of Babylon have brought to light the fact that the practice of punishing offenders against the law by throwing them into a den of lions was common.