(noun) - An ancient mode of confirming a bargain. In a bargain between two Highlanders, each of them wets the ball of his thumb with his mouth and joins them together, esteemed a very binding act. It was customary with these kings, in concluding a peace or striking an alliance, to join their right hands and bind their thumbs together. Immediately, when the blood had diffused itself to the extremities, it was let out by a prick and licked by the contracting parties. Their covenant was henceforth deemed sacred.
--John Jamieson's Etymological Dictionary of the Scottish Language, 1808