Hardcore

wiktionary

ref

hard +‎  core 1936 (n.); 1951 (adj.) 

etymonline

ref

hardcore

also hard-core; 1936 (n.); 1951 (adj.); from hard (adj.) + core (n.). Original use seems to be among economists and sociologists, in reference to unemployables. Extension to pornography is attested by 1966. Also the name of a surfacing material.