No one's asked this question yet, but I thought I might as well get a jump on it. The expression is common enough, but if you're not familiar with it, a back of the envelope calculation is a quick, simple calculation done as an estimate. It's called "back of the envelope" because it can be written out on a small sheet of paper. I've used the backs of envelopes, the fronts of envelopes, scraps of paper, even pencil-friendly tabletops. Generally these calculations include all sorts of estimates and outright guesses at the unknown variables as well as equations simplified to the point of unrecognizability, and the result is not considered reliable, just close enough to work with.
When I first applied for this address on blogspot, the idea was to name the blog after myself. My last name is Crankshaw, so possibilities included Crankshaw's Thoughts, Crankshaw's Meanderings, The Book of Crankshawian Philosophy, etc. The blog has superseded the homepage as the ultimate vanity project. Nothing really felt right, though, so I started thinking of other names, a name appropriate for an engineer writing about things he was distinctly unqualified to discuss. It took surprisingly little time to come up with "Back of the Envelope."