'somnolent' originates from Latin, specifically the word 'somnolentus,' where 'somnus' meant 'sleep.'

'somnolentus' in Latin passed into Late Latin/Medieval Latin as 'somnolent-' and was borrowed into English (via learned/Latin usage) as 'somnolent'.

Initially, it meant 'inclined to sleep' or 'sleepy,' and over time it has retained that basic meaning while also being used to describe things that induce or suggest sleep.