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.
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.