Contraction of 'has not'; the present perfect form of negation used with third-person singular subjects.
Contraction combining 'has' (Old English 'hæfde') and 'not' (Old English 'not'). Contractions became standardized in written English during the 18th century, though spoken contractions are much older.
Contractions like 'hasn't' are how we know language is alive—formal writing avoided them for centuries, but we needed them to sound natural, so eventually writing had to catch up to how humans actually talk.
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