a condition causing abnormal weakness of certain muscles.
(in full myasthenia gravis /ˈgrɑːvɪs, ˈgravɪs/) a rare chronic autoimmune disease marked by muscular weakness without atrophy, and caused by a defect in the action of acetylcholine at neuromuscular junctions.
mid 19th century: modern Latin, from Greek mus 'muscle' + asthenia
myasthenia in other Oxford dictionaries
Definition of myasthenia in the US English dictionary