To go off halfcocked

from The Collaborative International Dictionary of English v.0.48

Halfcock \Half"cock`\ (-k[o^]k`), v. t. [imp. & p. p.
   {Halfcocked}(-k[o^]kt); p. pr. & vb. n. {Halfcocking}.]
   To set the cock of (a firearm) at the first notch.
   [1913 Webster]

   {To go off half-cocked}, {To go off halfcocked}.
   (a) To be discharged prematurely, or with the trigger at half
       cock; -- said of a firearm.
   (b) To do or say something without due thought or care.
       [Colloq. or Low]
       [1913 Webster]