I've got a huge collection of PS slang and glossary type entries that I've been working on for several months, however there's a few terms that I haven't been able to pin down with a definition. Maybe one of you can help? 
The 3 undecided terms are:
1.) Creaker
2.) Scap
3.) Stony magic
Any help is appreciated.
I believe you're right about scap.
I found a few references to scap and planescape after some digging. It seems to indicate a messy action, with implications of destruction or wasting.
SCAP = Supreme Commander for the Allied Powers
It's a modern slan dictionary, but it does have an entry for scap:
http://www.urbandictionary.com/define.php?term=scap
At first I read #2 as "Scab", which I would have defined as a quick fix. Such as "That 'ole creaker scapped our gig, but we were able to scab it over long enough to beat feet."
As for Stony Magic, the obvious first reference might be any kind of protection or elemental stone magic.
A better definition might be ancient magic that has resided in an area for a great many years. As in "We thought we'd make quick work o' that old temple, but it was protected by Stony Magic. "
In the mind of adding to the vernacular, how about these two I came up with some time back?
1) "Don't Look Up" : A term used in much the same way as one might say "don't look back" - encouraging an individual to move on with their lives or their course of action. The phrase comes from the idea that Sigil sits at the "top" of the spire and a person in the outlands, second guessing his course of action might "look up" in regret.
2) "Don't Look Down" : A warning to a person not to hesitate, especially when doing so might cause them to fail. This is most commonly used when the individual in question is engaged in dangerous activities. The term originates from the misconception that a person "looking down" from the ring is likely to see the danger of the infinite drop and freeze up, falling to their doom.
Creaker: An old person, maybe another word for barmy as well? Or someone who can't keep their mouth shut?
Scap: Definately negative...Maybe to use as a verb for a messy action, or messing up something like..."We were going for a great peel, but then that ol' creaker scapped it"
Just my two cents