The word 'willful' is a compound formed within English from its native Germanic elements. Its first element, 'will,' descends from the Proto-Germanic wiljô, meaning "desire" or "wish," which itself traces back to the Proto-Indo-European root welh₁-, meaning "to wish, to will." The suffix '-ful' is an adjectival marker meaning "full of" or "having the qualities of." Thus, the literal construction "full of will" originally described someone possessing strong desire or intention. Over time, this meaning specialized, acquiring a negative connotation to characterize actions done deliberately against what is right or reasonable, or a person stubbornly insistent on their own way.

Structure: will (desire, intention) + ful (full of, characterized by) - will: From Old English will(a), meaning "desire, intention, determination." It is a Germanic root. - ful: A suffix from Old English -ful, meaning "full of" or "characterized by." It is used to form adjectives from nouns.