Is it ensure, insure, or assure?
small, strong European falcon, early 14c., from Anglo-French merilun, a shortened form of Old French esmerillon "merlin, small hawk" (12c., Modern French émerillon), from Frankish *smiril or some other Germanic source (cf. Old High German smerlo, German Schmerl "merlin"). Spanish esmerejon, Italian smeriglio also are Germanic loan-words.
In the legends of King Arthur, a magician who acts as Arthur's principal adviser.