'mirthfully' originates from Old English, specifically the word 'myrð'/'myrth' (Old English 'myrð' often written 'myrth' or 'mirthe'), where the root meant 'joy, pleasure'; the adjective-forming suffix '-ful' meant 'full of', and the adverbial suffix '-ly' formed the adverb.

'mirth' appeared in Old English as 'myrð' and passed through Middle English as 'mirthe' (or 'mirth'); the adjective 'mirthful' was formed by adding '-ful' to mean 'full of mirth', and later the adverb 'mirthfully' was formed by adding '-ly'.

Initially it referred to 'joy' or 'merriment' (the noun 'mirth'); over time the adjectival form came to mean 'full of merriment', and the adverb came to mean 'in a merry or amused manner'.