early 14c., "principal outer garment, tunic, kirtle," typically made of cloth and usually with sleeves, worn alone or under a mantle, from Old French cote "coat, robe, tunic, overgarment," from Frankish *kotta "coarse cloth" or some other Germanic source (compare Old Saxon kot "woolen mantle," Old High German chozza "cloak of coarse wool," German Kotze "a coarse coat"); the ultimate origin is unknown. Spanish, Portuguese cota, Italian cotta are Germanic loan-words.
Coats of modern form, fitted to the body and having loose skirts, first appeared in the reign of Charles II of England. Since the beginning of the eighteenth century the coat has been of two general fashions: a broad-skirted coat, now reduced to the form of the frock-coat ..., and a coat with the skirts cut away at the sides (the modern dress coat), worn now only as a part of what is called evening dress. [Century Dictionary, 1897]
As "garment worn suspended from the waist by women and children" from late 14c. (the sense in petticoat). Transferred late 14c. to "the natural external covering of an animal." Extended 1660s to "a thin layer of any substance covering any surface." Coat-hanger "clothes-hanger designed to facilitate the hanging of a coat" is from 1872. Coat-card (1560s) was any playing card which has a figure on it (compare face-card). It later was corrupted to court-card(1640s).
mid-14c., also simply coat (mid-14c.); originally a tunic embroidered or painted with heraldic armorial bearings (worn over armor, etc); see coat (n.) + arm (n.2) and compare Old French cote a armer. Sense transferred in Middle English to the heraldic arms themselves. Hence turncoat, one who put his coat on inside-out to hide the badge of his loyalty (1550s).
c. 1600, "flaps formed by the lower back of a coat," from coat (n.) + tail (n.). In 17c., to do something on one's own coattail meant "at one's own expense." Meaning "power of one person," especially in politics, is at least from 1848 (in a Congressional speech by Abraham Lincoln); expression riding (someone's) coattails into political office is from 1949.
But the gentleman from Georgia further says we [Whigs] have deserted all our principles, and taken shelter under General Taylor's military coat-tail, and he seems to think this is exceedingly degrading. Well, as his faith is, so be it unto him. But can he remember no other military coat-tail under which a certain other party have been sheltering for near a quarter of a century? Has he no acquaintance with the ample military coat-tail of General Jackson? Does he not know that his own party have run the five last presidential races under that coat-tail? And that they are now running the sixth under the same cover? Yes, sir, that coat-tail was used not only for General Jackson himself, but has been clung to, with the grip of death, by every Democratic candidate since. [Lincoln, speech in Congress, July 27, 1848]