`空口无凭` is an extremely common and practical idiom used across all levels of society.
Business and Negotiations: It is the unspoken rule in any negotiation. It's used to gently but firmly insist on a written contract (合同, hétong). A businessperson might say, “Of course, we trust you, but 空口无凭, let's sign a formal agreement to protect both our interests.”
Legal Matters: In any dispute, this phrase is used to dismiss claims that lack evidence. A lawyer will immediately ask for receipts, contracts, chat logs, or any physical proof, stating that 空口无凭.
Everyday Transactions: When you buy something valuable, you get a receipt (收据, shōujù) or a warranty card (保修卡, bǎoxiū kǎ). If a dispute arises later, the vendor can't deny the sale because you have proof. The underlying principle is `空口无凭`.
Formality and Connotation: The phrase is generally neutral and factual. It is not considered rude to say it, as it's seen as a statement of common sense rather than a personal accusation. It can be used in both very formal legal documents and in casual conversations among friends.