"explosive powder for the discharge of projectiles from guns," early 15c., from gun (n.) + powder (n.). The Gunpowder Plot (or treason or conspiracy) was a plan to blow up the Houses of Parliament on Nov. 5, 1605, while the King, Lords and Commons were assembled there in revenge for the laws against Catholics (see guy (n.2)).
Entries linking to gunpowder
mid-14c., gunne "an engine of war that throws rocks, arrows or other missiles from a tube by the force of explosive powder or other substance," apparently a shortening of woman's name Gunilda, found in Middle English gonnilde "cannon" and in an Anglo-Latin reference to a specific gun from a 1330 munitions inventory of Windsor Castle ("... una magna balista de cornu quae Domina Gunilda ..."). Also compare gonnilde gnoste "spark or flame used to fire a cannon" (early 14c.).
The identification of women with powerful weapons is common historically (such as Big Bertha, Brown Bess, Mons Meg, etc.).
Or perhaps gun is directly from Old Norse gunnr "battle." The word was perhaps influenced by or confirmed by (or possibly from) Old French engon, dialectal variant of engin "engine."
Meaning grew with technology, from cannons to firearms as they developed 15c.; popularly applied to pistols and revolvers from 1744. In modern military use the word is restricted to cannons (which must be mounted), especially long ones used for high velocity and long trajectory. Hence great guns (1884 as an exclamation) distinguished from small guns (such as muskets) from c. 1400. Meaning "thief, rascal" is from 1858. For son of a gun, see son. To jump the gun (1912, American English) is a figurative use from track and field. Guns "a woman's breasts" (especially if prominent) attested by 2006.
[G]un covers firearms from the heaviest naval or siege guns (but in technical use excluding mortars and howitzers) to the soldier's rifle or the sportsman's shotgun, and in current U.S. use even the gangster's revolver. In the other European languages there is no such comprehensive word, but different terms for the small or hand gun of the soldier or sportsman (even these, sometimes differentiated) and the heavy naval guns or artillery pieces .... [Buck, 1949]
"fine, minute, loose, uncompacted particles," c. 1300, poudre, "ash, cinders; dust of the earth;" early 14c. of any pulverized substance; from Old French poudre "dust, powder; ashes; powdered substance" (13c.), earlier pouldre (11c.), from Latin pulverem (nominative pulvis) "dust, powder" (source also of Spanish polvo, Italian polve; see pulverize).
The insertion of the unetymological -d- was common in French (compare meddle, tender (adj.), remainder). German has it as a doublet; Puder via French and Pulver from Latin. From mid-14c. specifically as "medicinal powder;" specialized sense of "gunpowder" is from late 14c. In the sense "powdered cosmetic," it is recorded from 1570s.
Powder keg "small barrel for holding gunpowder" is by 1820; the figurative sense ("something likely to explode easily") is by 1895. Powder room, euphemistic for "women's lavatory," is attested from 1936. Earlier it meant "place where gunpowder is stored on a warship" (1620s). Powder monkey "boy employed on ships to carry powder from the magazines to the guns" is from 1680s. Powder blue (1650s) was smalt used in laundering; as a color name from 1894.
The phrase take a powder "scram, vanish," is from 1920; it was a common phrase as a doctor's instruction, so perhaps the notion is of taking a laxative medicine or a sleeping powder, with the result that one has to leave in a hurry (or, on another guess, from a magician's magical powder, which makes things disappear).
Avis dropped an exhausted little heap onto her aunt's bed. She put her hand over her heart and said piteously, "Oh, Aunt Joyce, I mustn't ever do that again. My heart's going awful fast. I shall have to take a powder. Wasn't it fun though-" Avis' dark eyes flashed. [from "The Evolution of Avis" in The Connecticut School Journal, Jan. 9, 1902]
When the wife of your breast has confessed she has drest
On just triple the sum you allowed her,
And has run up long bills for her frocks and her frills—
Take a powder, my friend, take a powder.
[from "The Panacaea," in Punch, Dec. 14, 1901]
Powder in the wind (c. 1300, meaning powdered spices) was a Middle English image of something highly valued but flawed in some way that renders it impermanent or doomed to loss (of virtues without humility, etc.).