This topic is presented in "The Book of Days" for March 11. From Scott's "The Antiquary":
'From the remotest days of antiquity the practice of licking the thumb has always been regarded as a solemn pledge or promise, existing, according to Tacitus and other authorities, not only among the Goths, the Iberians, and the Moors, but which may be traced through successive periods even down to our own time.
Lord Erskine, in his Institutes, affirms that among certain of the lower ranks in Scotland, the final settlement of a bargain was always signalized by the "licking and joining of thumbs," and decrees are at this moment extant testifying to the legality of sales effected upon "thumb-licking," with this interpretation, "that the parties had licked thumbs at finishing the bargain." Relics of this ancient custom are still to be met with among the vulgar in Scotland, as also in those parts of Ulster where the inhabitants are of Scottish descent, the common observation between two gossips who ultimately agree upon a disputed point being, "We may lick thooms upo' that!"
In another aspect, licking the thumb appears to have implied a challenge or promise to be redeemed at some future opportunity, equally significant as was the casting of the gauntlet at the rival's feet of an earlier period, and from which no departure was possible. But from the days of chivalry down to the time of Shakespeare, and long after, the recognized form of challenge was universally that of biting the thumb, though many historians and commentators argue that this may have been intended merely as an insulting gesture. At the very rising of the curtain upon Romeo and Juliet, the feud between the adherents of the rival houses is introduced by one of them biting his thumb, which is construed by those on the opposite side as an intentional insult or challenge for another deadly broil. Thus Samson, on the Capulet side, tells his companion that he will bite his thumb at them, "which is a disgrace to them if they bear it," upon which Abram, of the Montague, demands, "Do you bite your thumb at us, sir?" Here Gregory, having hastily taken in a calculation of the opposing numbers, thinks fit to decline the challenge, and returns: "No, sir, I do not bite my thumb at you, sir, but I bite my thumb, sir." This evasive reply is, however, of little avail, for the onslaught becomes general, the fatal issue of which is only prevented by the timely arrival of the Prince, who commands them instantly to disperse, under penalty of their lives. Again, the poet Decker, in his Dead Term, wherein he gives us a lively description of the groups of gallants who daily distinguished themselves in the walks of old St. Paul's Churchyard, uses this expression:—" What swearing is there, what shouldering, what jeering, what biting of thumbs to beget quarrels;" and, says a writer in Chamber's Book of Days, the whole history of a quarrel seems to be detailed in this quotation:
We almost see the ruffling swash-bucklers strutting up and down St. Paul's Walk, full of braggadocio and "new turned oaths." At first they shoulder, as if by accident; at the next turn they jostle ; fiery expostulation is answered by jeering, and then, but not till then, the thumb is bitten, expressive of dire revenge at a convenient opportunity, for fight they dare not within the precincts of the Cathedral cliiuvli.
In a note to his Lay of the Last Minstrel, Sir Walter Scott likewise alludes to this custom, viz.:
To bite the thumb or the glove seems not to have been considered upon the Border as a gesture of contempt, though so used by Shakespeare, but as a pledge of mortal revenge,and proceeds to narrate an instance wherein a young gentleman of Cheviotdale discovered on the morning after a hard drinking bout that his glove had been bitten, and knowing that he must have quarrelled with some one, he instantly repaired to his late companions until he whom he had challenged presented himself, following which the two engaged in a duel, which proved fatal to the challenger. This incident occurred at Selkirk in 1721.
From Seldon's Titles of Honour we learn that kissing the thumb was formerly a characteristic of servility, the clergy, the rich, and the great being in receipt of this honour from the "tradesmen" who had the privilege of supplying their household requisites. This ceremony was performed at every interview: the tenant kneeling and clasping the hands of his lord, he kissed the thumbs ere he rose to depart. The custom was widespread on the Continent, and peculiar most of all to Dauphiny.
Omens and superstitions have been connected with the thumb equally with other material things. Shakespeare has several allusions, notably in Macbeth, Act IV. sc. i, where the second witch, anticipating the approach of the murderer of Duncan, says:
By the pricking of my thumbs
Someone evil this way comes,
an omen as characteristic as that of the tingling of the ears, by which we believe ourselves to be the topic of thought or conversation on the part of a distant acquaintance. Again, in the same play, Act I. sc. 2, the first witch thus foretells the manner of Macbeth's return from the seas—
Though his bark cannot be lost,
Yet it shall be tempest tossed,
and being questioned by her companion as to the stability of her knowledge, she returns, showing her withered thumb:
Here I have a pilot's thumb,
Wreck'd as homeward he did come.'