Seriously / for real.
As a question, マジ？ means "for real?" As an intensifier, マジで means "seriously" — マジで美味しい is seriously delicious. Very common among younger speakers and in casual speech.
Clipped from 真面目 (majime), meaning serious or earnest.
Casual. In a business setting the word is 本当に (hontou ni).
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