Necessary or required; something that is needed.
From Old English 'need' plus the suffix '-ful' (meaning 'full of' or 'characterized by'). This is an older English word that's less common in modern usage but still appears in formal or literary contexts.
The phrase 'the needful' was Victorian slang for money—as in 'I don't have the needful to buy that!' This shows how older English words had cooler slang meanings than we might expect. The word is still used in legal and formal documents today.
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