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.
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. Sense of "grow up, be reared" (in a clan, etc.) is late 14c.; meaning "form by education" is from mid-15c. Related: Bred; breeding.
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<a href="https://www.etymonline.com/word/thoroughbred">Etymology of thoroughbred by etymonline</a>
Harper, D. (n.d.). Etymology of thoroughbred. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/thoroughbred