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The primary meaning of the adjective hair-triggered is, of a firearm: equipped with a hair trigger—i.e., with a trigger so delicately adjusted that a very slight pressure on it discharges the firearm.
The following explanations are from A Dictionary of the Military Science: Containing an explanation of the principal terms used in mathematics, artillery, and fortification [&c.] (London: Baldwin and Cradock, and T. Egerton, 1830), by E. S. Norman Campbell [page 249]:
TRIGGER. A steel catch, which, being pulled, disengages the cock of a gun-lock, and causes the flint to strike the hammer. The difference between a hair and common trigger is this; the hair trigger, when set, lets off the cock by the slightest touch; whereas the common trigger requires a greater degree of force, and consequently its operation is retarded.
The earliest literal use of the adjective hair-triggered that I have found is from the following advertisement, from Sales by Auction, published in The Morning Post (London, England) of Monday 21st March 1803 [page 4, column 3]:
By Mr. CHRISTIE,
At his Great Room, in Pall-mall, on Tuesday, the 29th instant, at Twelve o’Clock,
[…] Fire-arms, particularly a pair of Pistols, hair-triggered by Wogdon, a pair of ditto by Lover.
By extension, hair-triggered also means: easily activated or set off, reacting immediately to the slightest provocation or cause. These are, in chronological order, the earliest figurative uses of this adjective that I have found:
1-: From The Trundle Letters. Letter the Thirteenth, published in the Commercial Journal and Family Herald (Dublin, County Dublin, Ireland) of Saturday 26th November 1853 [page 7, column 2]:
I hurried from the saloon, and proceeded to take possession of that berth which I had so judiciously secured on the previous morning. But oh! my dear Toby, pity your correspondent when he informs you that that place of repose, on which he expected to solace his wearied limbs, was already occupied by Major M‘Snapper (the sour passenger to whom you have been already introduced). I immediately called the steward, laid before him the case, but found it was perfectly useless to declare my “berth-right,” for that functionary declared with divers significant nods and winks that it was impossible he could interfere, and hinted that he had noticed the gentleman take a pocket-pistol to his nocturnal embrace. Further he declared in a confidential whisper that to the best of his opinion and belief the aforesaid fire-arm was hair-triggered, and moreover upon full cock. “Hair-triggered—ah!” I soliloquised. “Like master like man.” A more hair-triggered temper than has been awarded to Major M‘Snapper never made a home unhappy, I venture to say.
2-: From Mr. Hart’s Railroad and Wagon Road Act, published in the Daily Butte Record (Oroville, California, USA) of Wednesday 5th May 1858 [page 2, column 2]:
This act has already excited some discussion in the press of the county, and amongst citizens who are to be effected by the injustice and burdens it will impose upon the county, should it be approved by a majority of our citizens at the next election. We shall not refer to the many peculiar features of the act at the present time, but shall take occasion to do so hereafter, when we have time and space to publish the entire act, that the citizens of the county may understand its provisions, and be enabled to vote upon the double-barreled and hair-triggered measure as in their judgment may seem right and proper that they should.
3-: From Messrs Potter and Pryor, published in the Cleveland Morning Leader (Cleveland, Ohio, USA) of Wednesday 18th April 1860 [page 2, column 3]—reprinted from the Cincinnati Daily Commercial (Cincinnati, Ohio, USA):
As a speaker, he [i.e., Roger A. Pryor 1] is fiery, declamatory and pompous, and altogether too prone to rash personal expressions. The blood flies to his head, and he “flies off the handle.” He had not spoken two minutes when he made his first effort to address the House, before he had indicated in very exceptional terms, that he was a fighting man of the hair triggered description. He was not to be intimidated, he said, “by anything the gentleman, (Nelson, of Tennessee,) says, much less by anything he may dare do!” And an impression was at once made upon the judicious that while the young man had some fine qualities, and was disposed to fight at the drop of the handkerchief, he was sadly wanting in cool-headed judgement.
1 The U.S. journalist and lawyer Roger A. Pryor (1828-1919) was a member of the House of Representatives from Virginia from 1859 to 1861. Known for his fiery oratory, he was in favour of slavery and, later, of secession from the USA.
4-: From Virginia Redivivus, published in The Philadelphia Inquirer (Philadelphia, Pennsylvania, USA) of Wednesday 27th February 1861 [page 4, column 3]:
Ex-Governor White 2 is another incumbrance upon Virginia’s title to self-government. […] A hair-triggered genius, the ideal of an abstractionist 3, is the said Henry A., with less character and less capability as a conductor or counsellor, and more faculty for springing mines and exciting disturbance, than any man living; an agitator so subtle and sincere that if he were known to be insane he would be none the less indispensable as a Sensationist 4.
2 A prominent slave owner, Henry Alexander Wise (1806-1876), Governor of Virginia from 1856 to 1860, served as a significant figure on the path to the American Civil War (1861-65).
3 Here, the noun abstractionist designates a doctrinaire supporter or advocate of states’ rights, typically from one of the Southern states.
4 Here, the noun sensationist seems to designate a person who seeks to cause a strong emotional response by making use of sensational language.
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