What does this mean? I think it’s cockney rhyming slang, but I can’t figure it out. Butcher’s knife, block? Thanks in advance.
A “butcher’s” is a “butcher’s hook” and means a “look”. So “fancy a butcher’s?” would mean “Want a look?”
[Slightly related hijack]
I just finished the PS2 game Ghosthunter. During one sequence, my character interacted with the ghost of a WWII-era British naval commander named Admiral Fortesque.
Admiral Foresque had a rather colorful way of speaking. Describing when the Nazis boarded his ship, AF said something like “Those Nazi chaps tried to give my lads the ol’ slippity slappity, but my lads had the what now to give them they ol what for and a side of how’s your mamma. Jolly good and Bob’s your uncle.”
So as I was playing I was using an FAQ (commonly referred to as a cheat guide). The person who wrote the FAQ described AF like this: “Uh-oh. Looks like Admiral Fortesque bangs on a tad hey what.” Bolding mine.
I’m assuming the phrase “bangs on a tad hey what” is the FAQ author’s way of imitating AF’s gibberish. Or does that phrase actually mean something?
To “bang on” means to talk excessively and/or inanely, e.g. “What’s he banging on about?”.
“A tad” is “a bit”, often used ironically to mean “quite a lot actually”, as in “It’s getting on my nerves a tad.”
“What?” on the end of a sentence is just an old-fashioned affectation; it doesn’t really mean anything - think of it as like Canadian “eh” 
Usually associated with the “I say…” at the start of the sentence.
So basically the FAQ writer is saying that the admiral has a spot of the old verbal diarrhoea…
It’s obvious if you use your loaf.
I should coco!
Funny, I always thought Fancy had a somewhat different occupation.
Do Americans use the form the old, or is it strictly a Commonwealth thing?
The old as used is fine. Slightly dated cliche. “How’s the old complaint?” “He’s a chip off the old block.”