Is There Such a Word as Rhumb?

noun The short, thick first digit of the human hand, differing from the other fingers in having but two phalanges; the pollex.

Are thumb fingers?

A thumb is a digit, but not technically a finger. Many people don't make the distinction between thumbs and other digits.

Why do they call it thumb?

THUMB. ... The term “thumb” was first used before the 12th century and is believed to have come from the Proto-Indo-European term tum, meaning “to swell,” which makes the thumb "the swollen one." There is some debate as to whether the thumb can rightfully be called a finger, but classification aside, the name fits.