The Collaborative International Dictionary of English v.0.48:

Hagbut \Hag"but\ (-b[u^]t), n. [OF. haquebute, prob. a
   corruption of D. haakbus; haak hook + bus gun barrel. See
   Hook, and 2d Box, and cf. Arquebus.]
   A harquebus, of which the but was bent down or hooked for
   convenience in taking aim. [Written also haguebut and
   hackbuss.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

hagbut
    n 1: an obsolete firearm with a long barrel [syn: arquebus,
         harquebus, hackbut, hagbut]