Full of grapes or containing many grapes; abundant in grapes.
From English 'grape' (from Old French 'grape', originally meaning a cluster or hook-like tool) combined with the suffix '-ful' (meaning full of, abundant in). The grape itself likely derives from the tool used to harvest grapes.
The suffix '-ful' is one of English's most productive word-building tools—we can attach it to almost any noun to mean 'full of' that thing. But grapeful is rarely used because we just say 'a bunch of grapes' instead, showing how speakers naturally prefer existing words over logically possible ones.
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