Dating back to the Second World War (1939-45), and of American-English origin, the colloquial adjective trigger-happy means: over-ready to shoot at anything at any time or on slight provocation.
The adjective happy has been used as the second element in other compound adjectives, particularly common during and following the Second World War, relating to mental instability associated with the first element—as in demob-happy.
This use of happy is comparable to that of the adjective crazy in compound adjectives such as stir-crazy, meaning: deranged by long imprisonment.
It seems that the adjective trigger-happy was coined during the first major U.S. offensive against the Japanese, which took place, from August 1942 to February 1943, on and around Guadalcanal, one of the southern Solomon Islands, a large archipelago in the south-western Pacific.
The earliest occurrence of the adjective trigger-happy that I have found is from a report by Richard Tregaskis, of the International News Service (INS), published in the Morning World-Herald (Omaha, Nebraska) of Tuesday 13th October 1942:
Guadalcanal, Solomon Islands, Sept. 20 (Delayed) (INS)—Japanese army forces on Guadalcanal, defeated at the battle of Lunga Ridge, have pulled back into the jungle as far as 15 or 20 miles.
Several of our men who were marooned behind enemy lines during the rapid movements of the Lunga Ridge engagement, reported that the Japs were jittery, chattering nervously with one another and wasting many rounds of ammunition by “trigger happy” shooting at shadows.
The second-earliest occurrence of the adjective trigger-happy that I have found is from an article about the letters written by Private First Class Lee B. Morton, of the U.S. Marines in the Solomon Islands, published in the Moberly Monitor-Index and Moberly Evening Democrat (Moberly, Missouri) of Tuesday 29th December 1942:
Morton enclosed in a recent letter a parody on “The Old Grey Bonnet”, which a friend of his in the Marines wrote. The lines are:
Now when the sirens start to squawling [= squalling]
To my foxhole I’ll be crawling
And remain till break of day,
For I know there’s no mistaking
When the earth starts to shaking
That’s the safest place to stay.
Now we may be trigger happy
But we’re after Mr. Jappy
And we’ll get him soon, I know.
The earliest occurrence of the noun trigger-happiness that I have found is from a report by B. J. McQuaid, of the Chicago Daily News, from New Georgia, in the Solomon Islands, published in several U.S. newspapers on Friday 13th August 1943—for example in the Akron Beacon Journal (Akron, Ohio):
A COMMAND POST IN THE JUNGLE, New Georgia, July 28 (Delayed)—I heard the highest praise spoken by a general about his men, here in this post just behind the fighting front at Munda airfield, where I was interviewing him.
“They have behaved like old campaigners,” the general said. He and the leaders of the young fighters said all these troops were under fire for the first time, except for one unit which had fought the Japs on Guadalcanal.
There have been very few cases here of jungle war neurosis in comparison with certain other outfits which started in the New Georgia campaign. There have been no wild outbreaks of panicky shooting and trigger-happiness in the night. Each evening an airtight defense was thrown around our forces which made it easy to identify as an enemy anyone who moved about in the darkness.
The second-earliest occurrence of the noun trigger-happiness that I have found is from the title of an article published in The Flint Journal (Flint, Michigan) of Tuesday 30th November 1943:
Increased Deer Hunter Toll Laid to ‘Trigger-Happiness’
Lansing—AP—State conservation officers today attributed a record number of fatal shooting accidents among deer hunters this year to “trigger-happy” fellows more interested in getting a supply of venison to eke out their meat supplies than in the sport of hunting.