The Collaborative International Dictionary of English v.0.48:

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).
   [1913 Webster]

   Note: The sackbut of the Scriptures is supposed to have been
         a stringed instrument.
         [1913 Webster]

WordNet (r) 3.0 (2006):

sackbut
    n 1: a medieval musical instrument resembling a trombone

Easton's 1897 Bible Dictionary:

Sackbut
   (Chald. sabkha; Gr. sambuke), a Syrian stringed instrument
   resembling a harp (Dan. 3:5, 7, 10, 15); not the modern sackbut,
   which is a wind instrument.