lethargy: [14] Greek léthē meant ‘oblivion’ (the Romans used it for the name of a river in Hades whose water induced forgetfulness, and its influence has also been traced in changing Latin lētum ‘death’ to lēthum, source of English lethal [17]). From it was formed the adjective léthargos, which in turn produced the noun lēthargíā, source (via Latin and Old French) of English lethargy. => lethal[lethargy etymology, lethargy origin, 英语词源]