The phrase "hard shell" typically refers to someone who is tough or resistant, both physically and emotionally. It means they are not easily influenced or affected by outside forces and can be difficult to break or penetrate. Full definition
They will be sending out a complimentary premium in - vehicle charger and a slide - out hard shell case as a thank you gift to their customers. (androidheadlines.com)
The bran is the outer hard shell of the grain. (diabetes.org)
I have a thick hard shell on outside but very soft on the inside. (bikerdating.ca)