clothier n.

"woven fabric, pliable stuff made of intertexture of threads or fibers," Old English claþ "a cloth, sail, cloth covering, woven or felted material to wrap around one," hence, also, "garment," from Proto-Germanic *kalithaz (source also of Old Frisian klath "cloth," Middle Dutch cleet, Dutch kleed "garment, dress," Middle High German kleit, German Kleid "garment"), which is of obscure origin, perhaps a substratum word.

As an adjective, "made or consisting of cloth," from 1590s. Meaning "distinctive clothing worn by some group" (servants of one house, men of some profession or trade) is from 1590s, hence The cloth "the clerical profession" (1701).

word-forming element indicating "one whose occupation has to do with," from French and Old French -ier, from Latin -arius (see -ary). Nativized and used to form English words (glazier, hosier, etc.) Also see -yer, and compare -eer.

c. 1300, from Anglo-French tailour, Old French tailleor "tailor," also "stone-mason" (13c., Modern French tailleur), literally "a cutter," from tailler "to cut," from Late Latin or old Medieval Latin taliare "to split" (compare Medieval Latin taliator vestium "a cutter of clothes"), from Latin talea "a slender stick, rod, staff; a cutting, twig."

Although historically the tailor is the cutter, in the trade the 'tailor' is the man who sews or makes up what the 'cutter' has shaped. [OED]

The post-Latin sense development would be "piece of a plant cut for grafting," hence a verb, "cut a shoot," then, generally, "to cut." Possible cognates include Sanskrit talah "wine palm," Old Lithuanian talokas "a young girl," Greek talis "a marriageable girl" (for sense, compare slip of a girl, twiggy), Etruscan Tholna, name of the goddess of youth.

Kent. ... You cowardly rascal, nature disclaims in thee; a tailor made thee.

Corn. Thou art a strange fellow: a tailor make a man?

Kent. Ay, a tailor, sir: a stone cutter, or a painter, could not have made him so ill, though they had been but two hours at the trade.

["King Lear"]

One who makes outer garments to order, as opposed to a clothier, who makes them for sale ready-made. Tailor-made first recorded 1832 (in a figurative sense); literal sense was "heavy and plain, with attention to exact fit and with little ornamentation," as of women's garments made by a tailor rather than a dress-maker.