Is it ensure, insure, or assure?
c.1600, the instrument itself known from 14c. in English, from Late Latin syrinx, from Greek syrinx "shepherd's pipe." Used of vocal organs of birds from 1872.
syrinx syr·inx (sēr'ĭngks)
n. pl. syr·inx·es or sy·rin·ges (sə-rĭn'jēz, -rĭng'gēz)
A pathological tube-shaped cavity in the brain or spinal cord.