From Longman Dictionary of Contemporary Englishpin moneyˈpin ˌmoney noun [uncountable] MONEY British English a small amount of extra money which someone earns to spend on things which they want but do not really need She helped her uncle out sometimes just to earn a bit of pin money.
Examples from the Corpus
pin money• It helped when I used to do a bit of coal-bag carrying in my spare time to earn some pin money. 