(thinking of writing apologetic e-mail re still unfinished review, along the lines of 'sorry to be such a big girl's blouse', I wonder whether, as an American, I have really mastered the idiom, turn to our dear friends at Google and find....)
[Q] From Colin Alexander, New Zealand: “Big girl’s blouse. How did this extraordinary pejorative come about? It is usually applied to males and seems to mean a milquetoast, but how?”
[A] For those in other parts of the English-speaking world who have never heard of this astonishing idiom, let me explain that it is heard now quite widely in Britain (and elsewhere, too, it seems), though it originated in the North of England.
I’ve been vaguely dreading somebody asking this question, because it is one of a set of Northern idioms that are quite impenetrable in their origins. Others are the exclamation of surprise, “well, I’ll go to the foot of our stairs!” and the dismissive “all mouth and trousers”.
People do indeed use it to mean an ineffectual or effeminate male, a weakling, though it is often used in a bantering or teasing way rather than as an out-and-out insult (“You can’t drink Coke in a pub, you big girl’s blouse!”; “Blokes who don’t take on dares are big girl’s blouses”). The American milquetoast isn’t quite equivalent (since it has a greater emphasis on meekness rather than on an unmanly nature), but it’s close.
the rest here
(the consensus seems to be that it is some kind of put-down delivered to men, but I first came across it when my friend Sue McCafferty, who had grown up in the Lake District, said, 'Oooooooh [oo as in cool] babes, sorry to be such a big girl's blouse', and I said '¿Qué?')