this one I am familair because in my younger days, my grandma did explain to me about this expression and I was told not to say it because of the significant.
In Hokkien term, "wah Lau" is oh my father, or to the married woman is, oh my hubby. In the old days, this cry was heard only in funeral and especially when the coffin was lowered to be burried, and the children will all scream, wah lau, wah lau, wah lau ...... as loud as possible so that the dead father/husband can hear who scream the loudest (he is supposed to be the most filial) .
So this cry was carried forward to those tattoo guys, and when they are in disagreement, the chief always scream, >>>> wah lau, lim peh (your father) yao beh si (not dead yet) li ko kau peh kau boh.........
So the younger generation actually does not understand this true meaning of this word, and in every instance they kau peh kau boh >>> wah lau eh...