Wiktionary
n. (context music English), (''stringed instruments'') position when the thumb is contacting the string
Wikipedia
In music performance and education, thumb position, not a traditional position, is a string instrument playing technique used to facilitate playing in the upper register of the double bass, cello, and related instruments, such as the electric upright bass. To play passages in this register, the player shifts his or her hand out from behind the neck and curves the hand, using the side of the thumb to press down the string; in effect, the side of the thumb becomes a movable nut ( capo).
Usage examples of "thumb position".
Prandial Musings appeared in thumb position on the op-ed page, a good spot!
He also had a writing commitment: two features a week for the Moose County newspaper, to be given thumb position on page two.
It took skill of the highest order to adjust the squeeze and thumb position correctly, but Bigman had that.