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From the link:

The false etymology persists despite the Oxford English Dictionary definition: "A method or procedure derived entirely from practice or experience, without any basis in scientific knowledge; a roughly practical method." The OED dates the phrase's first reference to 1692.

In the Encyclopedia of Word and Phrase Origins, "rule of thumb" is additionally defined as a method by which brewers once tested the temperature of a batch of beer: They dipped a thumb in the brew.