From Paul Krugman’s NYT column yesterday (“The Comeback Skid”):
There will be two big stars at the Republican National Convention, and neither of them will be Mitt Romney. One will, of course, be Paul Ryan, Mr. Romney’s running mate. The other will be Chris Christie, the governor of New Jersey, who will give the keynote address. And while the two men could hardly look or sound more different, they are brothers under the skin.
How so? Both have carefully cultivated public images as tough, fiscally responsible guys willing to make hard choices. And both public images are completely false.
How so? is perfectly ordinary — a truncated idiom (glossed by OED2 as ‘How is it so? How is that?’, by NOAD2 as ‘how can you show that that is so?’) — but I hadn’t realized just how old it is: the OED‘s first cite is 14th century, and the cites go through several centuries from there (including one from Shakespeare: “How so sir, did she change her determination?”). The cites run out in 1632, which just means that this entry hasn’t been updated.
In any case: how is it so that the two men are brothers under the skin? (and then the answer follows).