hard-shelled
/ˈhɑːdʃɛld/adjective
1Having a hard shell or surface; = hardshell.
2Difficult to solve or penetrate; intractable. Now rare.
3Originally and chiefly US. Strongly or rigorously committed, uncompromising, especially in one's adherence to religious or political principles; hard-line; conservative.
4Hardened, insensitive, callous; not easily moved.
5Of a book: hardbacked. Somewhat rare.
Origin
Late 16th century; earliest use found in Thomas Moffett (1553–1604), physician and naturalist. From hard + shelled.