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]

The Collaborative International Dictionary of English v.0.48:

Hackbuss \Hack"buss\ (h[a^]k"b[u^]s), n.
   Same as Hagbut.
   [1913 Webster]