Contraction of 'could not,' meaning was unable to or did not have the ability to do something.
From Middle English 'could' (past tense of 'can'), which comes from Old English 'cunnan' meaning 'to be able,' plus 'not.' The contraction 'couldn't' became common in written English by the 1800s.
Contractions like 'couldn't' are actually ancient—people have been smooshing words together in speech for millennia, and written contractions with apostrophes are just us writing down what people always said naturally, making them grammatically correct despite looking casual.
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