E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.
Rule of thumb. In the legend of Knockmany Fin, Mr. Coul says:
That baste Cucullin [is coming], for my thumb tells me so. To which his wife replies: Well, my Cully, dont be cast down . Maybe Ill 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 oclock. In these cases the rule of thumb refers to the prognostics of the thumb, referred to by the witches of Macbeth. By the pricking of my thumbs, something evil this way comes.