A shortening of the phraze 'Glossy Topcoat' used to describe a clear coat to protects ones nails after they've bean painted. This word is mostly used by Cristine of Simply Nailogical, also known as the queen of Holo, when she is too lazy to pronounce the word 'Topcoat'.
Glossy taco is just simply(nailogical) a better way of saying glossy top coat which is a clear layer of lacquer that you apply over dried polish to create a barrier surface to prevent against chipping and provide a high-gloss shine that dries in a short amount of time.