In the symbolism of the hand, the thumb is the symbol of the Child. The index finger is the Mother and the middle finger is the Father. The thumb and first two fingers raised is the hand of blessing. "The yogi’s mystic gesture places the index finger and the thumb together, while the other three fingers extend and make the sign for a triad. This was once the expression of the unity of mother and child under the protection of the triple Goddess, connoting security, love, blessings and other fortunate qualities" (Walker, pg. 326). According to Hindu scriptures, each person's soul is a small manikin the size of a thumb, living inside the heart, performing in ongoing dance felt as the heartbeat. Walker writes, "This thumb-soul is ‘the lord of time, past and future’ ".
In Catholicism, the thumb is the symbol of the chief person of the Godhead. "The thumb, the master or phallus finger is assigned to Venus as bringer and increase" (Jobes, pg. 717). Seal rings were worn on the thumb and represented virility and authority. According to Cooper, "Power and transmission of power; the thumb upwards is beneficent power, good luck, and good will; downwards it is the reverse" (pg. 172).
Bite the thumb – a sign of contempt
Rule of thumb – measurement by thumb, hence any procedure based on instinct or practical experience and not on science
Under one's thumb – under another's influence or control
Vestal's thumbs – in Roman times, turned up or down to save or the gladiator’s life
photo from https://organicallyjamie.com
See Bibliography post for references.