1. An exclamation used to attract someone's attention quietly or discreetly, often used to call someone over without drawing attention from others
Psst! Come over here, I need to tell you something.
Psiu! Vem cá, preciso te contar uma coisa.
2. A hissing sound made to get someone's attention in a subtle manner
He made a psst sound to catch her attention across the room.
Ele fez um som de psiu para chamar a atenção dela do outro lado da sala.