I got a tattoo on my left instep.
The kanji and the word are exactly the same. That means they share meanings as well.
This word also has a few more meanings: instep (the top part of the arch in your foot) and the back of the hand. These are parts of you that kinda look like turtle shells in shape, don't you think?
This word is made up of a single kanji, but it uses the on'yomi reading, not the kun'yomi reading. Luckily, you've learned this reading already. Yay!