Let go of my thumb!
When I'm nervous, my thumbs shake.
I’ve got a callus on my thumb.
Your parent finger is your thumb! It's bigger than your other fingers, so that means it's in charge too! This goes for the fingers on your feet too, I mean, your toes. So the parent toe is your big toe.
This word uses the kun'yomi readings from both kanji, probably because it includes body parts. You know them both already, so now you know this too!