The Insult was a statement made by Captain Jeffry L. Rand to the VUX, poisoning the relationship between the VUX and humans forever. Hayes says that it happened in 2119, while the SC2 manual says the event happened in 2126, which also conflicts with other elements in the Hayes' summary of the Ur-Quan Slave War.
Although reports of the encounter and the Insult are hazy, it is generally agreed Captain Rand, surprised by the images of a VUX captain, made some kind of derogatory statement concerning VUX appearance (and possibly even the VUX captain's parentage). According to one account, upon first seeing the VUX captain, Rand said "That’s the ugliest freak-face I’ve ever seen!" While Rand had not supposed the VUX to hear or understand his inadvertent statement, the far superior VUX translation technologies unfortunately delivered every nuance of that statement to the VUX captain. Being an extremely vain race, every VUX that heard of this statement (which was quickly all of them) immediately began to hate every human with an inexpressible loathing that lasts to this day.
Another account states that the Insult was Rand joking with his lieutenant that VUX stood for "Very Ugly Xenoform."
It is believed that the Insult is one of the reasons why the VUX gladly joined the Hierarchy instead of the Alliance. Some in the Alliance even had hopes that the effects of the Insult could be reversed following the end of the Ur-Quan Slave War. In reality, the VUX use the Insult merely as an ostensible reason for their hatred, covering a true and deep disgust for the human appearance.