Big Uglies was a slang term for humans in the Race's language. The obvious definition of the slang was that humans are "bigger" and taller than the Race (who are around the size of an average 10-year old human child) and look "ugly" to them (seeing that the Race were reptilian beings or "Lizards", and humans were mammals). It was invented by members of the Conquest Fleet during the war against major human powers, set off when they invaded and attempted to conquer Earth.
Initially, certain humans found the term offensive. Teerts, a captive of Japan in 1942-3, was heavily beaten for saying it. After the hostilities ended, it went from being a slur to simply being a nickname (much like the words "Yanks" or "Limeys"), and was generally not considered to be very offensive. General George Patton commented: "Beauty is in the eye of the beholder, or so they say. In my eye, Lieutenant, those are ugly sons of bitches, and if they think me ugly, well, by God, I take it for a compliment.”