The word 'zestful' is a relatively modern English formation, built upon the noun 'zest'. 'Zest' entered English in the 17th century from French 'zeste', which referred to the piece of citrus peel used as a flavoring. The French term likely originated from an unattested Latin word 'zestum', perhaps related to Greek 'zōstēr' (a belt or girdle), metaphorically describing the peel encircling the fruit. The semantic evolution moved from the literal peel, to the flavor it imparts, and finally to the figurative sense of a piquant quality that adds excitement or enjoyment. By adding the native English suffix '-ful', meaning 'full of', the adjective 'zestful' was created to describe someone or something brimming with that lively, enthusiastic quality.

Structure: zest (lively quality) + ful (full of) zest: Origin: English (from French 'zeste'). Functional role: The core noun meaning a piquant flavor or spirited enjoyment. ful: Origin: Old English '-ful'. Functional role: An adjective-forming suffix meaning 'full of' or 'characterized by'.