This expression almost certainly originated in the letterpress print shop. In many typecases lower case p was close to lower case q,* and when distributing type back into the case the apprentice would need to be on his toes with these two sorts. This was especially so because on the piece of type the letter would appear reversed, so p would look like q and q would look like p.

The Oxford Words blog gives some alternative derivations, which are basically those given in The Oxford English Dictionary. Interestingly they do not include the printing origin, although The Oxford Companion to the Book plumps wholeheartedly for it. A foolish consistency is of course the hobgoblin of little minds. “With consistency a great soul has simply nothing to do.” And who would deny Oxford a great soul?

It looks like this may be one of those occasions when we get to form the plural with an apostrophe. The OED does it; the Oxford Words blog does not; the Companion does. Just do whatever you want!


* I know that this isn’t the case with the California job case, almost universal in America. Obviously its designers strove proactively to mind their ps and qs.