"condition in which the extremities perform slow, involuntary motions" (a form of childhood cerebral palsy), 1871, with -osis + Greek athetos "not fixed, without position or place, set aside," from athetein "to set aside, reject as spurious," from a- "not" (see a- (3))+ tithenai "place, set" (from reduplicated form of PIE root *dhe- "to set, put"). Coined by U.S. nerve specialist William Alexander Hammond.