There are several theories about the origin of "rule of thumb" and I don't think the matter is settled. My favorite theory is the one described in Gulliver's Travels, by Jonathan Swift, where the narrator describes how the Lilliputians made a shirt for him:

"The sempstresses took my measure as I lay on the ground, one standing at my neck, and another at my midleg, with a strong cord extended, that each held by the end, while the third measured the length of the cord with a rule of an inch long. Then they measured my right thumb, and desired no more; for by a mathematical computation, that twice round the thumb is once round the wrist, and so on to the neck and the waist, and by the help of my old shirt, which I displayed on the ground before them for a pattern, they fitted me exactly."