'watertightly' originates from English, specifically the compound word 'watertight' + the adverbial suffix '-ly', where 'water' comes from Old English 'wæter' meaning 'water' and 'tight' meant 'secure, closely fastened'.

'watertight' developed in English as a compound of 'water' + 'tight' (early Modern English); the adverbial form 'watertightly' is formed by adding the suffix '-ly' (from Old English '-lice' / Middle English '-ly') to the adjective 'watertight', producing the modern adverb.

Initially it described something physically 'secure against water'; over time the word has kept that core sense and 'watertightly' means 'in a way that prevents water from entering'.