'myasthenia' originates from New Latin, specifically the combination of 'myo-' meaning 'muscle' and 'asthenia' meaning 'weakness.'

'myasthenia' was formed in the late 19th century from the Greek roots 'mys' (muscle) and 'astheneia' (weakness), and became the modern medical term 'myasthenia.'

Initially, it meant 'muscle weakness,' and this meaning has remained consistent in modern medical usage.