phrase usage - What does "How come" mean?

Is How come a phrase? What does it mean? Is it formal or informal? British or American?
Can I use it in anywhere?

Answer

How come is a fixed phrase with fixed uses of its own.

1. 
   

   It means, as you have divined, why, and may be used, like why, either as the head of a noun phrase, or as a bare interrogative, or as an interrogative head; but as an interrogative head it does not take ‘DO-support’—that is, it is followed by an ordinary clause in indicative form:

   
   

   
   

   And that's why I went to the party. ... And that's how come I went to the party.
   Why? .... How come?
   Why did you go to the party? ... How come you went to the party?

   
   

   
   


2. 
   
   

   It is never inflected: we never say ^✲How came or ^✲How has come or ^✲How comes or anything of the sort.

   
   

How come is not used in formal writing or speech; it is distinctly casual.