'willful' originates from Old English elements: the noun 'will' (from Old English 'willan') meaning 'wish, desire' and the suffix '-ful' (from Old English 'ful') meaning 'full of'.

'willful' appeared in Middle English as 'wilful' (with variable spelling) and eventually became the modern English adjective 'willful'; the adverb 'willfully' developed by adding the adverbial suffix '-ly'.

Initially it meant 'full of will' or 'having strong desire'; over time it shifted to emphasize deliberate or intentional action, giving the modern sense 'intentionally' for 'willfully'.