Ankle
noun
What is the neck of your leg? Think about it. It's your ankle.
This word uses the kun'yomi readings from both kanji, probably because it includes body parts. You know the kun'yomi reading of 足 from previous vocab and you know the kun'yomi reading of 首 from learning that kanji. So, you should be able to read this kanji so long as you remember that it's all kun'yomi.
足首のストレッチをしましょう。
Let's stretch your ankle.
ハイヒールをはいて、足首がいたくなった。
I wore high heels and my ankles became sore.
大きい山でハイキングするときは、足首をケガしないようにね。
When you hike on a large mountain, don't injure your ankle.