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"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 (cf. Old English eaxl, Old Saxon ahsla, Old High German ahsala, German Achsel "shoulder"), from PIE *aks- "axis" (see axis). Found only in compound axle-tree before 14c.