Is it ensure, insure, or assure?
Old English deor "animal, beast," from Proto-Germanic *deuzam, the general Germanic word for "animal" (as opposed to man), but often restricted to "wild animal" (cf. Old Frisian diar, Dutch dier, Old Norse dyr, Old High German tior, German Tier "animal," Gothic dius "wild animal," also cf. reindeer), from PIE *dheusom "creature that breathes," from root *dheu- (1) "cloud, breath" (cf. Lithuanian dusti "gasp," dvesti "gasp, perish;" Old Church Slavonic dychati "breathe").
For prehistoric sense development, cf. Latin animal from anima "breath"). Sense specialization to a specific animal began in Old English (usual Old English for what we now call a deer was heorot; see hart), common by 15c., now complete. Probably via hunting, deer being the favorite animal of the chase (cf. Sanskrit mrga- "wild animal," used especially for "deer"). Deer-lick is first attested 1778, in an American context.