"weakness, debility," 1788, medical Latin, from Greek astheneia "want of strength, weakness, feebleness, sickness; a sickness, a disease," from asthenes "weak, without strength, feeble," from a- "not, without" (see a- (3)) + sthenos "strength, power, ability, might," which is of uncertain origin, perhaps from PIE root *segh- "to have, hold," on the notion of "steadfastness, toughness."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sahate "he masters, overcomes," sahah "power, victory;" Avestan hazah "power, victory;" Greek skhema "figure, appearance, the nature of a thing," related to skhein "to get," ekhein "to have, hold; be in a given state or condition;" Gothic sigis, Old High German sigu, Old Norse sigr, Old English sige "victory."
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Definitions of myasthenia from WordNet
a chronic progressive disease characterized by chronic fatigue and muscular weakness (especially in the face and neck); caused by a deficiency of acetylcholine at the neuromuscular junctions;