1. have an adverse result, or the opposite effect to what was intended
the plan/decision backfired
When something 'backfires' it means that a plan or action goes completely wrong or has the opposite effect to what was expected. This negative outcome is often caused by taking risky action, or by misjudging the situation with faulty information or guesswork. When something 'backfires', it all goes wrong. A similar expression might be 'blow up in your face', which also means something going wrong, creating a negative outcome for you and going against your expectations.