Green’s Dictionary of Slang

1. (US) a member of the primitive Baptist Church; thus softshell, Baptists who are less severely fundamentalist.

1845		W.T. Porter Big Bear of Arkansas (1847) 62: He lived with his father, an old ‘hard-shell’ Baptist preacher.
1867		G.W. Harris Sut Lovingood’s Yarns 183: A hard-shell preacher wif his mouf mortised into his face in shape like a muel’s shoe. [Ibid.] 187: That durn’d hiperkritikil, groanin ole Hardshell raskil.
1871		E. Eggleston Hoosier School-Master (1892) 134: Some people think the ‘Hardshells’ a myth, and some sensitive Baptist people at the East resent allusions to them.
1896		Sun. School World Oct. 340/1: The Hard-shells have a church six miles from here on the top of the Blue Ridge [DA].
1897		(con. 1860s) W.E. Barton Hero in Homespun 2: Preacher Tate is a Hardshell. He hates a fiddle wus’n pizen.
1908		L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:iv 319: Hardshell, n. A Primitive Baptist.
1927		(con. 1917) J. Stevens Mattock 311: My pa, he’s a Hard-shell [...] his uncle’s still a Tennessee Hard-shell elder.
1934		E. Caldwell Tobacco Road (1958) 42: Bessie hated Hard-shell Baptists with the same intensity with which she hated the devil.
1987		J. Ciardi Good Words 143: Hardshell [...] A strictly fundamentalist Baptist.

2. (US) an uncompromising conservative person.

1853		N.Y. Clipper 8 Oct. 3/5: [T]he hard shells had a meeting in the Park.
1860		Northampton Mercury 24 Nov. 4/6: Few Englishmen can distinguish between a ‘hard shell’ and a ‘soft shell’ and [...] are puzzled to understand how in Democratic Republic political parties can be divided into Republicans and Democrats.
1882		‘Paris Inside Out’ in Nat. Police Gaz. (NY) 23 Dec. 6/3: ‘Give me a good figure and well turned legs. Eh, old hard shell?’.
1899		A. Binstead Gal’s Gossip 23: Her father [...] might have picked up the art of trading in Middlesex Street itself; he was a real bit of hard shell!
1912		Van Loan ‘Behind the Mask’ in Ten-Thousand-Dollar Arm 235: They expected to see the old hardshell begin to ‘guess’.
1916		H.L. Wilson Somewhere in Red Gap 135: A grouchy old hardshell with white hair.
1987		J. Ciardi Good Words 143: Hardshell [...] Generic label for anyone of adamant conviction.