Short for "texas university," t.u is a name that followers of Texas A&M use to describe the University of Texas. Such a nickname implies that UT is a university in Texas rather than THE University of Texas, no matter what the Texas state Constitution might say. Additionally, they don't capitalize it because, they claim, UT isn't important enough to be capitalized, but in doing, they're actually denigrating their own state, so the non-capitalization most likely stems from the fact that the vast majority of Aggies happen to be illiterate. As a result, it becomes one of the few insults in the world that disrespects the insulter's own state and its constitution while simultaneously making the insulter appear uneducated and illiterate (a common byproduct of a Texas A&M "education"), while doing virtually nothing to actually insult the insultee.
Agnes the Aggie: "Eww, you're going to t.u."
Larry the Longhorn: "Aww, that's cute... Even a Sooner could come up with a more coherent insult than that! By the way, I hope you're not too upset about not getting into UT."