An exlamation used by visitors of a pop or rock concert, and sort of a phonetic derivation of the more classic We want more!
, which is used to ask the band for an encore, and is essentially an expression of praise for the band. This new derivation is a result of two things.
1) When a large concert hall full of people shout "We want more!" together, it usually mixes up to a fuzzy wall of noise which sounds something like "We wa wo!", almost sounding like an animalistic cry or a war chant. Some people have now jokingly stopped using the original alltogether, and started shouting the phonetic version of it.
2) Bands coming back after being asked for more used to be an exclusive treat for an extraordinarily enthousiastic crowd, but it has now become a fixed ritual at the end of +90% of all pop and rock concerts, leaving many people wondering why the band doesn't simply play the songs included in the encore in the first place, without going off stage first. Therefore, amongst some frequent concert visitors, the "We want more!" has evolved into the more lazy bastardization "We wa wo!"
*band leaves the stage after explicitly promising to come back if the crowd shouts loud enough*
Crowd: "WE WA WO! Yadda, yadda, break time is over - get on with it, dammit!"