From small +‎ fortune (“one’s wealth; the amount of money one has, especially if it is vast; large amount of money”), suggesting a sizeable sum of money but not all of one’s wealth.

• (Received Pronunciation) IPA^(key): /smɔːl ˈfɔːtʃuːn/

• Audio (Southern England):

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• (General American) IPA^(key): /smɔl ˈfɔɹtʃən/, /-tʃun/
• Hyphenation: small for‧tune

small fortune (plural small fortunes)

1. (colloquial, hyperbolic) A large amount of money, especially one paid for something.

   Synonyms: bundle, (both informal) mint, (slang) pile

   Divorcing my wife cost me a small fortune.

• “small fortune” under “fortune, n.”, in OED Online 👁 Paid subscription required
  ⁠, Oxford: Oxford University Press, 1897.