Tacitus reports, that amongst certain barbarian kings their manner was, when they would make a firm obligation, to join their right hands close to one another, and intertwist their thumbs; and when, by force of straining the blood, it appeared in the ends, they lightly pricked them with some sharp instrument, and mutually sucked them.
Physicians say that the thumbs are the master fingers of the hand, and that their Latin etymology is derived from pollere. The Greeks called them ἀγτίχειρ, as who should say, another hand. And it seems that the Latins also sometimes take it in this sense for the whole hand:
Sed nec vocibus excitata blandis,
Molli pollici nec rogata surgit.
Neither to be excited by soft words or by the thumb. · Mart., 12.98.8
It was at Rome a signification of favour to depress and turn in the thumbs,
Fautor utróque tuum laudabit pollice ludum;
and of disfavour to elevate and thrust them outward,
converso pollice vulgi,
Quemlibet occidunt populariter.
The populace, with inverted thumbs, kill all that come before them. · Juvenal, 3.36
The Romans exempted from war all such as were maimed in the thumbs, as having no more sufficient strength to hold their weapons. Augustus confiscated the estate of a Roman knight who had maliciously cut off the thumbs of two young children he had, to excuse them from going into the armies; and, before him, the Senate, in the time of the Italic war, had condemned Caius Vatienus to perpetual imprisonment, and confiscated all his goods, for having purposely cut off the thumb of his left hand, to exempt himself from that expedition.
Some one, I have forgotten who, having won a naval battle, cut off the thumbs of all his vanquished enemies, to render them incapable of fighting and of handling the oar.
The Athenians also caused the thumbs of the Æginatans to be cut off, to deprive them of the superiority in the art of navigation.
In Lacedaemon, pedagogues chastised their scholars by biting their thumbs.