The amount of something that can fill one carton or cardboard box.
From 'carton' (from French carton, from Italian cartone, from Latin charta meaning 'paper') plus '-ful' (from Old English ful, meaning 'full'), indicating capacity.
The word 'cartonful' treats 'carton' like any noun that can hold stuff—similar to 'cupful' or 'handful'—showing how English speakers productively attach '-ful' to container nouns to talk about quantities without needing exact numbers.
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