The amount that can be held or contained within a cage; as much as a cage can hold.
Compound word formed from 'cage' (from Old French 'cage') plus the suffix '-ful,' meaning 'as much as will fill.' The pattern is standard in English for capacity measurements.
Words ending in '-ful' are clever linguistic shortcuts—instead of saying 'the amount of something that fills a container,' we compress it into one word, but the -ful reminds us it originally meant literally full.
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