(covers information from several alternate timelines)
"I'm a barbarian. You said so yourself."
The term barbarian referred to an individual who acted with a rash destructive nature, often involving loot and pillage as a means to achieve their aims. On Earth, the term was originally used in Ancient Greece.
When Captain Braxton was stranded on Earth in the late-20th century, he referred to the residents as being "post-industrial barbarians." (VOY: "Future's End") Later, in describing the experience to Seven of Nine, he stated "After three decades with those post-industrial barbarians I had to go through extensive rehabilitation before I could return to duty." (VOY: "Relativity")
When a group of Triannons seized control of Enterprise NX-01, their leader, Pri'Nam D'Jamat, stated that it might sound barbaric to the Enterprise crew, but it didn't matter how many heretics died. (ENT: "Chosen Realm")
When several crewmembers from the ISS Enterprise were accidentally beamed to the USS Enterprise, Commander Spock found it easy to identify them, stating to Captain Kirk that "It was far easier for you as civilized men to behave like barbarians, than it was for them to behave like civilized men." (TOS: "Mirror, Mirror") In 2370, Intendant Kira Nerys referred to 23rd century mirror universe Terrans as barbarians. (DS9: "Crossover")
In 2364, the Anticans referred to Humans as "barbaric" upon learning that Humans ate replicated meat. (TNG: "Lonely Among Us") That same year, Victoria Miller referred to the Betazoid wedding ritual as a "barbaric ceremony". (TNG: "Haven")