The contraction of 'are not,' written without an apostrophe as sometimes appears in informal writing or representing how it sounds in speech.
From 'are' (plural of 'to be') and 'not,' combined into a contraction. The spelling 'aren't' with an apostrophe is standard, while 'arent' represents informal or dialectal writing.
Contractions like 'aren't' are interesting because they're technically informal—but English speakers use them constantly in real speech, so formal writing that avoids them sounds stiff and artificial.
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