In Orwell's A Clergyman's Daughter, Dorothy ends up travelling with a bunch of other homeless youths, one of whom is a cockney called Nobby.
He uses the word "Mulligatawny" as a slang word, but I've never heard it used this way anywhere else:
After Dorothy hands over her last coin:
'That's the mulligatawny!' he said. 'We've struck it lucky--and so've you, kid.
After she tells him her name:
'Ellen. That's the mulligatawny. No surnames when you're on the bum.'
When he's describing about going to pick hops to make some money:
‘Pickin’ ’ops—dahn in Kent! C’n understand that, can’t yer?’
‘Oh, hops! For beer?’
'That's the mulligatawny! Coming on fine, she is. Well, kid, 'z I was saying, here's us three going down hopping, and got a job promised us and all--Blessington's farm, Lower Molesworth. Only we're just a bit in the mulligatawny, see? Because we ain't got a brown between us, and we got to do it on the toby--thirty-five miles it is--and got to tap for our tommy and skipper at night as well. And that's a bit of a mulligatawny, with ladies in the party.'
From the above, he seems to be using the word in a couple of different ways — to express that he finds something agreeable (similar to "now that's what I'm talking about" or "that's the ticket"), or to confirm that Dorothy is understanding him correctly ("Now you're getting it"), but finally he seems to use the word to mean 'a tricky situation' or 'in a pickle'.
Does this usage have an origin outside the book?