at half cock
1. Of a gun, with its hammer pulled back half way (as opposed to "full cock," in which the hammer is positioned as far back as possible in order to allow it to be fired with minimal pressure on the trigger). He wasn't going to shoot—his gun was only at half cock.
2. By extension, hastily or impulsively, without proper preparation. I'll admit that I went off at half cock when I sent that angry email before finding out what really happened.
at half cock
when only partly ready. informal At half cock is used of a firearm with the cock lifted but not moved to the position at which the trigger will act. It is usually found in go off at half cock or go off half-cocked meaning ‘go ahead without making proper preparation and therefore fail’.