origin of the phrase 'rule of thumb'

At a workshop today about edible weeds, we discussed the danger of tasting unknown plants, and the group agreed that it would be generally safe to try a tiny piece of foliage from a plant. (But I guess it would be most unfortunate if anyone tried that on a poisonous toadstool.)

One participant said he'd heard the phrase 'rule of thumb' originates from the idea that it's fairly safe to try a piece of leaf that's smaller than a man's thumb.

I can't see any references to this origin on the Internet. In fact, it seems more likely that the phrase comes from the use of the thumb as a rough measure of length. Wikipedia points out:

 The use of a single word or cognate for "inch" and "thumb" is common in many other Indo-European languages, for example, French: pouce inch/thumb; Italian: pollice inch/thumb; Spanish: pulgada inch, pulgar thumb;Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb; Sanskrit: angulam inch, anguli finger; Slovak: palec, Slovene: palec inch/thumb, Czech: palec inch/thumb.

By the way, it was a great workshop. I as I've said before and before and before, our thinking often depends on the words we use. If I call this lovely little plant a weed, it might irritate me to see it growing in my garden.

But if I call it a herb, or even a potherb, or just plain old 'food', then it's a different story. I think it's chickweed, but I'm not sure enough to say so straight out. I'd better sign up for another weed workshop, or perhaps I should consult Doris Pozzi's handbook of edible weeds. (She presented the workshop.)