"pole or pin upon which a wheel revolves," Middle English axel-, from some combination of Old English eax and Old Norse öxull "axis," both from Proto-Germanic *akhsulaz (cognates: Old English eaxl, Old Saxon ahsla, Old High German ahsala, German Achsel "shoulder"), from PIE *aks- "axis" (see axis). Found only in compound axletree before 14c.