The Brief: The phrase "metal" or "that's metal" means that something is hardcore, much like metal music.
“Metal” can be used in direct reference to metal music, or metaphorically for something that resembles the musical genre in its hardcore-ness.
a my chemical romance reunion announcement on halloween? that’s metal.
— Morgan Guidry (@mo_arianna) October 31, 2019
That's metal af (no pun intended) https://t.co/LZILt8smXN
— 𝖕𝖊𝖓𝖉𝖗𝖆𝖌𝖔𝖓.☾ (@pendragontwt) October 30, 2019
“We stand on guard for thee?” That’s metal af and makes me want to charge into battle. And I’m not even Canadian!
— Clay Winstead (@ClayWinstead) October 31, 2019