'mouthwateringly' originates from Modern English, specifically the word 'mouthwatering', where the suffix '-ly' meant 'in the manner of'.

'mouthwatering' developed as a compound of 'mouth' + 'watering' in Modern English; 'mouth' comes from Old English 'mūþ' and 'water' comes from Old English 'wæter' (from Proto-Germanic *watar), and the adverb was formed by adding the productive adverbial suffix '-ly' to the adjective.

Initially it was used quite literally to mean 'causing the mouth to water (produce saliva)'; over time it has been used more broadly and figuratively to mean 'extremely appetizing or enticing', and 'mouthwateringly' now means 'in a way that causes mouth to water; deliciously'.