Properly written: ¡Ay, güey!
It is a very informal
Mexican exclamation, similar to "OH, SHIT!" without the intense level of vulgarity.
It comes from: Ay! - literal translation of exclamation in
Spanish of "OH!/ Ouch!"
Güey! - refers to "dude /
chav" hence why it is informal.
Ay wey, tu jefa esta bien buena. - "Oh,
dang! Your mom is so
hot."
Ay wey! Casi me caigo! - "Oh, shit! I almost fell down!"
¡Ay, wey! Ya no tengo dinero. - "Oh dang. I don't have anymore
money!"
Ay, wey. Se me pasó el camión. - "Oh, shit. I missed the
bus."
— ¡Ay, güey! — Exclamó Eratóstenes. - "Oh
fuck!" Exclaimed Eratosthenes.
**Either spelling is equivalent to mean the same thing, except "ay wey" is more prevalent in informal writing or among anyone basically (refer back to the fact of being an informal expression). "¡Ay, güey!" is more prevalent in literature but is not exclusive.