Goroawase is a kind of wordplay using words based on the sound of numbers. A famous example is the word for the Japanese mafia, yakuza, which comes from yattsu (eight), kyu (nine), san (three). Hanafuda is a traditional Japanese card game where the worst possible hand combination consists of an eight, a nine and a three. The yakuza are the worst of the worst.
That's from an interview with my friend, writer Aidan Doyle.