"a jaw, a jawbone," 1670s, from Latin maxilla "upper jaw," diminutive of mala "jaw, cheekbone." "Maxilla stands to mala as axilla, 'armpit,' stands to ala 'wing'" [Klein]. Especially a bone of the upper jaw (maxilla superior) as distinguished from the mandible or lower jaw (maxilla inferior). Related: Maxillar; maxilliform.
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.
"pole or pin upon which a wheel revolves" (properly, the round ends of the axle-tree which are inserted in the hubs or naves of the wheels), 1630s, from Middle English axel-, from some combination of Old English eax and Old Norse öxull "axis," both from Proto-Germanic *akhsulaz (source also of Old English eaxl "shoulder," oxta, ohsta "armpit," which survived as dialectal oxter; also Old Saxon ahsla, Old High German ahsala, German Achsel "shoulder"), from PIE *aks- "axis" (see axis, which is from the Latin cognate of this Germanic word). Found only in compound axle-tree before 14c.