From Longman Dictionary of Contemporary Englishtrigger-happytrigger-ˌhappy (also trigger-ready) adjective informal DANGEROUSsomeone who is trigger-happy is much too willing to use weapons, especially guns a trigger-happy cop
Examples from the Corpus
trigger-happy• A debate even begins about whether the army is not getting a little trigger-happy in its nervous and no doubt terrified tension.• It ensured that no trigger-happy missile controller would fail to observe the safety precautions and attempt a little target practice.• Whooper swans are molested by irresponsible trigger-happy parties but here, at Bolam, they visit in peace.• The investigator said the killings were the result of poor procedure by trigger-happy police rather than premeditated murder.• After all, half-an-hour away in any direction there is a trigger-happy racist waiting for his big day.• trigger-happy soldiers 