Miado is actually the way Brazilians describe the sound made by cats, which would be the English "meow". In this context, however, it is used when something is lame like a party that is actually not very nice. It can also be used when your friends cancel the plans they made with you.
"Que festa miada, cara! Vamos embora!"
"A gente ia pro bar mais tarde, mas meus amigos miaram o rolê."
"What a meowed party, man! Let's get out of here!"
"We were going to the bar later, but my friends meowed it."