go off half-cocked

From the days of flintlock and caplock firearms, where the half-cock position of the hammer was both a rudimentary safety, and the proper position for priming the pan or inserting a percussion cap. The phrase was originally rendered, "to go off at half-cock."

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go off half-cocked

1. (idiomatic, colloquial): To take a premature or ill-considered action.

   Make sure none of your men go off half-cocked and ruin this operation.

• flash in the pan
• lock, stock and barrel