Rule of Thumb
(The). A rough guess-work measure. Measuring lengths by the thumb. In some places the heat required in brewing is determined by dipping the thumb into the vat.
“`That baste Cucullin [is coming]. ... for my thumb tells me so.' To which his wife replies: `Well, my Cully, don't be cast down. ... Maybe I'll bring you better out of this scrape than ever you could bring yourself by your rule of thumb [referring to the pricking of the thumb].' ” —W. B. Yeats: Fairy Tales of the Irish Peasantry, p. 270.
Again, p. 274, Fin knew by the “pricking of his thumb” that the giant Cucullin would arrive at two o'clock. In these cases the “rule of thumb” refers to the prognostics of the thumb, referred to by the witches of Macheth. “By the pricking of my thumbs, something evil this way comes.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894