Short for mythomania or mythoman
Mythomania = A compulsion to embroider the truth, engage in exaggeration, or tell lies.
A mytho = a lie, or a lier
Mohamed : "Aw yesterday I was wit Jessica Alba, She wanted to sleep wit me, and i tell her no I have my gurl"
Kareem : "U're A MYTHO"
Sirel : "Yesterday I was wit my boi he told me he luv me"
DalysS : "it's a mytho ! He tell u dat, but I saw him wit J-Lo"