1701, of persons, "thoroughly accomplished," from thorough + past tense of breed. In reference to horses, "of pure breed or stock," from 1796; the noun is first recorded 1842.
Entries linking to thoroughbred
thorough adj.
c. 1300, adjectival use of Old English þuruh (adv.) "from end to end, from side to side," stressed variant of þurh (adv., prep.); see through. Related: thoroughly; thoroughness.
breed v.
Old English bredan "bring (young) to birth, procreate," also "cherish, keep warm," from West Germanic *brodjan (source also of Old High German bruoten, German brüten "to brood, hatch"), from *brod- "fetus, hatchling," from PIE root *bhreu- "to boil, bubble, effervesce, burn." The etymological notion is incubation, warming to hatch.
The intransitive sense "come into being" is from c. 1200; that of "beget or bear offspring" is from mid-13c. Of livestock, etc., "procure by the mating of parents and rear for use," mid-14c. The sense of "grow up, be reared" (in a clan, etc.) is late 14c.; the meaning "form by education" is from mid-15c. Related: Bred; breeding.