A core is the hard middle of a thing: the part you cannot bite through and cannot squeeze. Hard-core can be pictured as that middle, laid over people and over material. The fans who are still there in the rain are the hard middle of a crowd, the part the bad season could not wear away. Material called hard-core is the middle too, in a different way: the part with nothing taken out and nothing covered over.
Two branches
一群人裡最硬、最不會動搖的那一塊
這是最貼近核心畫面的用法：把「咬不動的那塊核」直接說成一群人。
adjective · B2
Used in front of a noun to describe people whose beliefs or way of living are extreme and very unlikely to change, and also the small group inside a larger group who are the most committed of all.
死忠的、極端的
什麼都不遮的那一端（只活在固定說法裡）
同一個「最硬、沒有包覆」的畫面，換成講內容的強弱程度。
adjective · C1
Hard-core pornography means films, magazines or websites that show sexual acts in full and open detail, with nothing hidden; the expression often carries the idea that the material is crude or unpleasant.
露骨的色情內容