'lethargic' originates from Late Latin 'lethargicus', which comes from Greek 'lēthargikos', derived from 'lēthargos' meaning 'forgetful'.

'lēthargikos' transformed into the Late Latin word 'lethargicus', and eventually became the modern English word 'lethargic'.

Initially, it meant 'forgetful or inactive', but over time it evolved into its current meaning of 'sluggish and apathetic'.