egafd.com

"JJ, Pedants' College Cambridge."

JJ, Google Research Institute, writes:
No luck- and I'm not prepared to wade through several pages of Interwebby tommyrot on the off-chance, on a stomach-full of Guinness and no din-din.
My guess, 4WIW, is it's a reference to the diamond-trade.

I believe that it comes from tailoring.

The more cloth "wasted" in seams the less profit, so tailors were taught to cut the cloth fine (that is to say as close to the chalk mark as possible).

The resultant material left over for seams was sometimes so narrow it left little room for manouevre!

If you wish, I can also tell you the origin of "OK" and the V sign, too! ) hehehe

Much more plausible.