O tempora o mores! is a famous sentence by Cicero in his First Oration against Catiline.
It translates as Oh the times! Oh the customs!
In his opening speech against Catiline, who had previously tried to kill him, Cicero deplores the viciousness and corruption of his age. Cicero is frustrated that, despite all of the evidence that has been compiled against Catiline, who has been conspiring to overthrow the Roman government, and the fact that the senate has given senatus consultum ultimum, Catiline has not yet been executed. Cicero goes on to describe various times throughout Roman history where consuls have killed conspirators with even less evidence. Sometimes, in the case of former consul Lucius Opimius' slaughter of Gaius Gracchus (one of the Gracchi brothers) based only on "quasdam seditionum suspiciones" certain suspicions of insurrection.
This sentence is now used as an exclamation to criticize present-day attitudes and trends, often jokingly or ironically.