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.
"He must be a tea-sip. He has a picture of old 'taco meat' in his room."