'wouldn't' originates from English, specifically the phrase 'would not', where 'would' is the past form of 'will' (Old English 'wolde', from Proto-Germanic *wiljaną) and 'not' is the negation.

'would' changed from Old English 'wolde' (past of 'willan') and combined with 'not' to form the negative phrase 'would not'; the contracted spoken form 'wouldn't' developed in modern English and became common in informal usage.

Initially, 'would not' expressed past willingness/volition or the negative of 'will'; over time the contracted form 'wouldn't' has been used widely for refusals, conditional/hypothetical negation, and negative habitual past.