Walleu was developed in the Vietnamese society in Aarhus-Denmark. It's a twisted contraction of the Arabian word "walla" ( = I swear ) and Vietnamese "deo" ( = fuck). The term is used as an exclamation in the same way as "shit", "damn" or "fuck".
A guy sees a hot chick in the street and poke his friend:
"Look! Walleuuu, she hot".
Guy to girl on the dancefloor: "Walleu, nice ass baby wanna come home with me?"
guy A: "I saw your mom yesterday with another guy".
guy B: "Walleu?"