A casual, shortened form of 'another'; used in informal speech as in 'a whole nother level.'
American informal speech created by the process called 'reanalysis'—speakers heard 'an other' and reanalyzed it as 'a nother,' which then became 'nother.' This is not standard in written English but is very common in spoken American English.
The word 'nother' shows how English keeps evolving in speech before it's written down. It started as an accident of pronunciation (dropping the 'a'), but now millions of people use it naturally. Language rules we learn in school are just documenting what people already do!
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