Where did the term "money does not stink" come from?

Pecunia non olet is a Latin saying that means "money does not stink". The phrase is ascribed to the Roman emperor Vespasian (ruled AD 69–79).

A tax on the disposal of urine was first imposed by Emperor Nero under the name of “vectigal urinae” in the 1st century AD. The tax was removed after a while, but it was re-enacted by Vespasian around 70 AD in order to fill the treasury.

Vespasian imposed a urine tax on the distribution of urine from Rome's public urinals (the Roman lower classes urinated into pots, which were later emptied into cesspools). The urine collected from these public urinals was sold as an ingredient for several chemical processes. It was used in tanning, wool production, and also by launderers as a source of ammonia to clean and whiten woollen togas. The buyers of the urine paid the tax.

The Roman historian Suetonius reports that when Vespasian's son Titus complained about the disgusting nature of the tax, his father held up a gold coin and asked whether he felt offended by its smell. When Titus said "No," Vespasian replied, "Yet it comes from urine".

The phrase "pecunia non olet" is still used today to say that the value of money is not tainted by its origins.