Source : Webster's Revised Unabridged Dictionary (1913)

Sackbut \Sack"but\, n. [F. saquebute, OF. saqueboute a sackbut,
   earlier, a sort of hook attached to the end of a lance used
   by foot soldiers to unhorse cavalrymen; prop. meaning, pull
   and push; fr. saquier, sachier, to pull, draw (perhaps
   originally, to put into a bag or take out from a bag; see
   {Sack} a bag) + bouter to push (see {Butt} to thrust). The
   name was given to the musical instrument from its being
   lengthened and shortened.] (Mus.)
   A brass wind instrument, like a bass trumpet, so contrived
   that it can be lengthened or shortened according to the tone
   required; -- said to be the same as the trombone. [Written
   also {sagbut}.] --Moore (Encyc. of Music).

   Note: The sackbut of the Scriptures is supposed to have been
         a stringed instrument.

Source : WordNet®

sackbut
     n : a medieval musical instrument resembling a trombone