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 'Avtixeip', 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., xii. 98, 8.]

It was at Rome a signification of favour to depress and turn in the
thumbs:

"Fautor utroque tuum laudabit pollice ludum:"

["Thy patron will applaud thy sport with both thumbs"
--Horace.]

and of disfavour to elevate and thrust them outward:

"Converso pollice vulgi,
Quemlibet occidunt populariter."