1. An onomatopoetic interjection used to attract someone's attention quietly or discreetly, often made by sucking air between the teeth and lips
He made a psiu-psiu sound to call his friend over without disturbing the class
Ele fez um psiu-psiu para chamar seu amigo sem perturbar a aula
2. A whispered or hushed sound used to summon someone subtly
The waiter gave a psiu-psiu to get the busboy's attention
O garçom deu um psiu-psiu para chamar a atenção do ajudante