used as a negative argument, expressing disbelief for any situation or sentence - not to use around formal people, unless very close to you (as in a business meeting with some
old partners, for
example).
Can also be used as a synonim to beat someone really
hard.
O
cara é rico? Rico o cacete! Ele não tem nada! - That guy, rich? No
fucking way! He ain't got nothing!
Você é o chefe aqui? O cacete! Zé Maluco é que é o chefe! - You're the boss here? My ass! Zé Maluco is the Boss!
Se ele não parar, vou dar um cacete nesse
cara! - If he doesn't stop, I'm gonna beat the guy's face up!
A policia chegou e desceu o cacete! - The cops arrived and beat everybody down!