You could use the "appropriate" definition, which is that it's a term coined by Cristine Rotenberg from Simply Nailogical, but we're all dirty here. A glossy taco is, for lack of better words, a wet pussy. Taco emoji=sext for pussy, glossy=wet, meaning that there's the aftereffects of orgasm or cum on the pussy.
Wow, you have a really glossy taco!