A jokingly derogatory term Aggies use for Bevo
, the longhorn mascot of the University of Texas. Indicates that the creature (and the school, by extension) is of exceptionally poor quality and is only fit to be ground-up and served as taco meat.
This term became especially popular after an ESPNU commercial depicting an Aggie playing a Pictionary/charades-like game. The visual prompt included UT's longhorn logo and a hook. The aggie refused to answer "Hook 'em Horns" and instead refers to the longhorn (somewhat disgustedly) as taco meat.
The video can be found online quite easily.