'well-rested' originates from English, a compound of 'well' and the past participle 'rested' (from 'rest'). 'well' comes from Old English 'wel' meaning 'well', and 'rest' comes from Old English 'ræstan' meaning 'to rest'.

'rest' changed from Old English 'ræstan' to Middle English 'resten' and eventually became the modern English word 'rest'. 'well' changed from Old English 'wel' to modern English 'well'. The compound 'well-rested' developed in modern English as a descriptive adjective.

Initially, it meant 'having rested well' (a literal combination of 'well' + 'rested'), and over time it has retained that meaning as 'having had enough rest; refreshed'.