The Brief: The phrase "metal" or "that's metal" means that something is hardcore, much like metal music.

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Variations of “that’s metal” include “that’s so metal,” “now that’s metal,” “that’s metal AF,” and simply “metal.” Synonyms for metal include bada**, savage, or wicked.

“Metal” can be used in direct reference to metal music, or metaphorically for something that resembles the musical genre in its hardcore-ness.