I know there are a few threads on the go discussing Sigil's Cant, but I want to focus this thread on one particular aspect of the Cant, namely Rhyming Slang.
From what I've read, the only 'rule' of rhyming slang is that you replace the word to be obscured with the first word of a phrase that rhymes with that word. Somethimes, apparently, another word of the phrase is used, and at other times the the replacement word itself is altered.
Problem with most of the rhyming slang dictionaries is that the slang uses english references that don't translate to Planescape. For example, britneys = Britney Spears = beers. That won't work in the Cage 'cause (Thank the Lady) nobody knows who Britney Spears is. Berk has already become entrenched, so we can't really worry about the fact that there's no 'Berkley' in Planescape. "Donald Ducked" doesn't translate well, either, though I think we can all guess what it means.
Here are a few to get started...
Lemon = lemon and lime = Prime
Bread = bread and butter = Cutter (alternate to bread = money)
Brace = brace and bit = Shit
Parsley = parsley and sage = Mage
There was a whole schtick on my thread about using baked goods to represent magic, based off "tarts" for "subtle arts". It's crappy rhyming slang but makes for an amusing set of metaphors.
Other ones... lessee... it's hard thinking of conjunctive phrases that would exist in Planescape...
Lords = Lords & Ladies = Hades (or the Grey Waste in general)
)
Ups = Ups & Downs = Gowns (or dresses in general; makes for interesting combinations with "high-ups")
Envy = Envy & Sloth = 'Loth (yeah, that'll make you popular
Bells = Bells & Whistles = Missiles (presumably of the siege variety, or maybe everyone's favorite 1st level spell)
beard = beard & stubble = trouble
Dark = dark & shady = Lady (plus, she is the ultimate dark...)
dog = dog & pony = cony (i.e. a victim)
Ladies = Lady's Blades = shades
Vrock = Vrock's screech = teach
Waste = Waste grub (aka larva) = bub [hence, get wasted. ;)]
And then, a few oddballs...
Hulla = Hullaballoo = 'zu == baatezu
King = king & pawn = 'gon == baatezu (for cornugon, etc.)
Lock = lock & key = 'ri == tanar'ri
Dying = dying breath = 'leth
...
Eh. Most of these suck but it's useful spadework for the real geniuses to come along and show us what's what