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Has had multiple definitions over the years, it seems. Here’s one:

“Originally a small allowance given to a woman in order to purchase clothes etc. for herself. More recently it is used to describe any small amount of money which might be earned by children or the low-paid for some service.”  (Source: https://www.phrases.org.uk/meanings/pin-money.html)

And then here’s a short article about some of the others and how they might have come up: http://www.word-detective.com/2013/07/pin-money

And that’s only super quick research, so who knows if any of it’s right. 🙂