The Collaborative International Dictionary of English v.0.48:

Sordine \Sor"dine\ (? or ?; 277), n. [It. sordina, sordino, from
   sordo deaf, dull-sounding, L. surdus. See Surd.] (Mus.)
   See Damper, and 5th Mute.
   [1913 Webster]