the thumb or big toe: the largest digit of the hand or the foot
Hebrew uses this one word for the single body part English has to name two different ways. On the hand it is the thumb; on the foot it is the big toe (the great toe). The word itself does not choose between them: it points to the thick first digit of a limb, and the text tells you which limb by adding "of the hand" or "of the foot."
Almost every time the word appears, it is part of a fixed ritual gesture. When priests were ordained, and when a person recovering from a serious skin disease was declared clean, blood and oil were dabbed on three points of the body: the right earlobe, the thumb of the right hand, and the big toe of the right foot. Touching the far edges of the body (ear, hand, foot) stood for setting apart the whole person, from head to foot, for God.
The one grim exception is the story of a defeated king whose thumbs and big toes were cut off. Removing exactly those digits crippled a warrior's grip and stride, so the mutilation was a deliberate way of ending his power to fight or rule.
A broader word, אֶצְבַּע, covers any finger or toe; this word singles out the biggest one.