It says that "Immense!" was a common dude expression in 1883 -- it doesn't say for what, exactly, but I presume appreciation. I like it, actually! I'm going to start using it and see if I can start a trend.
I don’t know the true origin, but it sounds like Scots to me. “Aw” = “all”, and “but aw” would presumably be like “after all”, or “though” in Bristol (or maybe Yorkshire?) dialects.
I'd like to imagine it means "I know, right?"