'lethargy' originates from Greek, specifically the word 'lēthargia,' where 'lēthē' meant 'forgetfulness' and 'argia' meant 'inactive.'

'lēthargia' transformed into the Latin word 'lethargia,' and eventually became the modern English word 'lethargy' through Old French 'lethargie.'

Initially, it meant 'forgetfulness and inactivity,' but over time it evolved into its current meaning of 'sluggishness and apathy.'