1706, "hip-joint," from Latin coxa "hip," which, according to de Vaan, is from PIE *koks-h- "limb, joint," and is cognate with Sanskrit kaksa-, Avestan kasa- "armpit," Old Irish coss "foot." As the first joint of the leg of an insect, crustacean or arachnid, by 1826. Cox for "thigh" was used in medical writings from c. 1400. Related: Coxalgia, coxitis.
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