the.com/mustard greens

means The peppery, dark-green leaves of the mustard plant, cooked low and slow in Southern kitchens or quickly stir-fried for a sharp bite.

from The 'mustard' part traces back through Old French 'moustarde' to Latin 'mustum,' the unfermented grape juice once mixed with ground mustard seeds to make the condiment — so the word originally named a sauce, not a plant. English borrowed it for the plant family itself, and 'greens' simply did what it always does in American cooking: turn any leafy vegetable, boiled with seasoning, into a noun of its own.