1550s, from turn (v.) + coat (n.). The image is of one who attempts to hide the badge of his party or leader. The expression to turn one's coat "change principles or party" is recorded from 1570s.
Entries linking to turncoat
turn v.
late Old English turnian "to rotate, revolve," in part also from Old French torner "to turn away or around; draw aside, cause to turn; change, transform; turn on a lathe" (Modern French tourner), both from Latin tornare "to polish, round off, fashion, turn on a lathe," from tornus "lathe," from Greek tornos "lathe, tool for drawing circles," from PIE root *tere- (1) "to rub, turn." Transitive sense in English is from c. 1300. Related: Turned; turning.
Use in expression to turn (something) into (something else) probably retains the classical sense of "to shape on a lathe." To turn up "arrive, make an appearance" is recorded from 1755. Turn about "by turns, alternately" is recorded from 1640s. To turn (something) loose "set free" is recorded from 1590s. Turn down (v.) "reject" first recorded 1891, American English. Turn in "go to bed" is attested from 1690s, originally nautical. To turn the stomach "nauseate" is recorded from 1620s. To turn up one's nose as an expression of contempt is attested from 1779.
Turning point is attested by 1640s in a figurative sense "point at which a decisive change takes place;" literal sense "point on which a thing turns; point at which motion in one direction ceases and that in another or contrary direction begins" is from 1660s.
coat n.
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).
coat of arms n.
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).