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The word shawarma comes from the Turkish çevirme, 'turning', after the rotating spit.
Thin-shaved layers of marinated chicken or lamb are stacked on a vertical rotisserie, shaved to order, and rolled into flatbread with garlic sauce, pickles, and fries. In Saudi cities it's sold from small storefront counters and roadside kiosks that stay open late into the night.
Why it's made this way: The vertical spit design traveled from the Levant and Turkey with migrant workers and traders, and it caught on in Saudi Arabia because it let one cook serve a constant stream of customers fast, with the meat self-basting as it turns.