thumb
拇 centers on the thumb, the thickest and most opposable finger of the hand.
拇 combines 扌 (hand) with 母, which likely contributes the sound and relates to the thumb as the 'mother' finger of the hand.
The hand radical 扌 beside 母 (mother) suggests the thumb as the most important finger, like a mother to the other fingers.
For ボ, imagine a bow tied around the thumb: bow -> ボ, and the thumb is the finger that helps tie it.