Why it says "cool"?

Popularize in the eighties and nineties, cool became a wildcard word to mean one thing or one person were amusing, amazing, fantastic and attractive.
Recently, word has been added to a certain burden to include it in the pejorative term "Go for cool," is said about who is or who falsely claims to money and fame.

But cool is a very old word that originally meant the Interjection ay. Thus, it had meaning of regret or threat: Guay Me!, Instead of Woe to me!; Guay or if you leave me!, Instead Woe to you if I leave!

Its use is already reflected in texts of the Middle Ages and can find the word without any effort at La Celestina (1499), by Francisco de Rojas.
Guay purchased the current meaning in the twentieth century to coincide phonetically with English voice gay (Funny, brilliant).