From Middle English reyndere, reynder, rayne-dere, from Old Norse hreindýri (“reindeer”), from hreinn (“reindeer”) + dýr (“animal”). Compare Dutch rendier (“reindeer”), German Rentier (“reindeer”), Swedish rendjur (“reindeer”), Danish rensdyr (“reindeer”) and French renne (“reindeer”). Related also to displaced Old English hrān (“reindeer”). Unrelated to rein.