When I read this word in Perdido Street Station, I assumed it was a misprint; but I looked it up, and sure enough, "skewwhiff" is an adjective or adverb from British dialect, meaning "askew" or "awry."

Interestingly, almost all of first few pages of Google search results for this term are definitions of it; it's not until the third or fourth page of results that actual uses of the term in text start to appear, such as a press release that refers to "this skewwhiff logic, where one set of rules would exist for one set of companies, while others were excluded[...]."