Plural form of cupful using the archaic/formal rule where the main noun (cupful) gets pluralized rather than the measure word.
From cupful + -s plural. This represents the older English convention of pluralizing the head noun in compound measurements, now considered overly formal or incorrect.
Grammar nerds love debating this: is it 'cupfuls' or 'cupfulfuls'? In Middle English, you'd pluralize the main word, making 'cupfulfuls' technically correct but hilariously awkward-sounding today!
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