In old gambling and underworld slang, queer means counterfeit money. The usage comes from the word’s earlier sense of strange or not genuine, and it survives in phrases such as shoving the queer, meaning passing fake notes off as real ones. At a casino, queer money might turn up at the cage, in a table buy-in or in the note acceptor of a slot machine.
Outside this narrow slang, the word has a very different meaning: it is widely used today as an identity term within the LGBTQ+ community, and it has also been used as a slur, so it calls for care. For operators, the underlying risk is real: cashiers, note acceptors and cage procedures need counterfeit checks, while online the equivalent threats are stolen cards and payment fraud, handled by risk controls.