Green’s Dictionary of Slang

1. one who is considered the most serious, the most dedicated.

1959		H. Salisbury Shook-Up Generation (1961) 89: There are twenty thousand families, constituting less that 1 per cent of the population of the city, which are the source of 75 per cent of all delinquency. This is what an earlier social studies cliche would call a ‘hard core’.
1963		M. Braly Shake Him Till He Rattles (1964) 10: He told Carver about a new meeting place, where the hard-core of the really hip hung out.
1974		G.F. Newman You Flash Bastard 164: Of all the villains we’re claiming, the majority of those going down have no previous. Time and gain the hard-core, the villains we know’re responsible for the villainy are getting results.
1995		G. Small Ruthless 227: Me lick me head [fraternised] wid me bad brother a Waterhouse, and from deh so me just start to be a hardcore.
2001		N. Barlay Hooky Gear 8: Duanes ex-ABA an KO’d some hardcore in his time.

2. the strongest varieties of pornography, usu. featuring uncensored still or moving pictures of intercourse, plus such personal preferences as paedophile shots, bestiality, extreme sado-masochism etc.

1973		Listener 27 July 120: California is busy withdrawing the liquor licences of strip-joints and bars putting on sex-shows. Hard-core may fetch the customers, but hard liquor brings in the profits.
1998		G. Burn Happy Like Murderers 104: The music from porno movies. Feature-length, hard core.