It's a set phrase used to express astonishment.
In Valencia the word 'baina' (also beina) can be used as synonymous of 'bother'.
Other example of this used is:
He, uncle pod!
Val. «El tio baina!»
It's used to reproach somebody for something. Most of the time it also implies that the 'baina' person has acted cunningly, that he is a cheeky devil or that he has nerve to to something. Thus, this expression would be a mild reproach, and sometimes even a positive amazement.