[1] : 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
see also:
sack butt sagbut 

[2] : WordNet (r) 2.0

sackbut
     n : a medieval musical instrument resembling a trombone

[3] : 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.
   

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Sackbut, definition of term: Sackbut
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