thumb, big toe

拇 is composed of the radical of 手 (hand) and 母 (mother). Since 母 refers to a woman who has given birth and extends to mean the main, primary, or source element, 拇 combines this with the hand radical to represent the largest, thickest, and most essential digit of the hand—the thumb. Later, its meaning was also extended to refer to the big toe on the foot.