What's the meaning of 'If you'll believe me' in this context? It's from a novel Baker's Blue Jay Yarn by Mark Twain.
Well, at last he could hardly flop his wings, he was so tuckered out. He comes a-drooping down, once more, sweating like an ice pitcher, drops his acorn in and says, 'Now I guess I've got the bulge on you by this time!' So he bent down for a look. If you'll believe me, when his head come up again he was just pale with rage.
I guess that means 'I'm not sure if you believe what I'm going to say or not'.