1610s, "loose outer garment" (worn by men in Eastern countries or in ancient times), from French veste "a vest, jacket" (17c.), from Italian vesta, veste "robe, gown," from Latin vestis "clothing," from vestire "to clothe" (from PIE *wes- (2) "to clothe," extended form of root *eu- "to dress"). The sleeveless garment worn by men beneath the coat was introduced by Charles II in a bid to rein in men's attire at court, which had grown extravagant and decadent in the French mode.
The King hath yesterday, in Council, declared his resolution of setting a fashion for clothes .... It will be a vest, I know not well how; but it is to teach the nobility thrift. [Pepys, diary, Oct. 8, 1666]
Louis XIV of France is said to have mocked the effort by putting his footmen in such vests.
"clothe again," with or as with a garment, c. 1300, revesten, from Old French revestir, from Late Latin revestire "to clothe again," from re- "back, again" (see re-) + vestire "to clothe, dress, adorn" (see vest (v.)). Related: Revested; revesting.
"trousers, drawers," 1840, see pantaloons. The word was limited to vulgar and commercial use at first.
I leave the broadcloth,—coats and all the rest,—
The dangerous waistcoat, called by cockneys "vest,"
The things named "pants" in certain documents,
A word not made for gentlemen, but "gents";
[Oliver Wendell Holmes, "Urania: A Rhymed Lesson," 1846]
Colloquial singular pant is attested from 1893. To wear the pants "be the dominant member of a household" is by 1931. To do something by the seat of (one's) pants "by human instinct" is from 1942, originally of pilots, perhaps with some notion of being able to sense the condition and situation of the plane by engine vibrations, etc. To be caught with (one's) pants down "discovered in an embarrassing condition" is from 1932.
"chaste, pure, virgin," 1590s, originally (early 15c.) "belonging to or dedicated to Vesta," Roman goddess of hearth and home, from Latin vestalis. The noun is recorded from 1570s, short for Vestal virgin, one of four (later six) priestesses (Latin virgines Vestales) in charge of the sacred fire in the temple of Vesta in Rome. From 1580s in reference to any virgin or chaste woman.
They entered the service of the goddess at from six to ten years of age, their term of service lasting thirty years. They were then permitted to retire and to marry, but few did so, for, as vestals, they were treated with great honor, and had important public privileges. Their persons were inviolable, any offense against them being punished with death, and they were treated in all their relations with the highest distinction and reverence. A vestal who broke her vow of chastity was immured alive in an underground vault amid public mourning. There were very few such instances; in one of them, under Domitian, the chief of the vestals was put to death under a false charge trumped up by the emperor. [Century Dictionary]