go off half cocked(also go off at half cock) to do something without enough thought or preparation, so that it is not successful
[TahlilGaran] Dictionary of Contemporary English ▲
go off half cocked v. phr.,
informal To act or speak before getting ready; to do something too soon.
Bill often goes off half cocked. Mr. Jones was thinking about quilting his job, but his wife told him not to go at half cock. [TahlilGaran] English Idioms Dictionary ▲