Anything at all; also means zero or nothing in some dialects.
From Old English 'awiht' or 'awuht' meaning 'anything' (a- prefix + wiht meaning 'creature' or 'thing'). Over time it could mean either 'anything' or its opposite, 'nothing'—a confusion that created two opposite meanings for the same word.
Aught is a perfect example of how homophones and sound-alikes can destroy a word's meaning—since it sounds like 'ought' (should), people got confused, and now 'naught' and 'nothing' mostly replaced it, except in old phrases like 'for aught I know.'
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