A stain is a mark or discoloration, and to stain something is to mark it with dirt or color it, either deliberately or accidentally. If you stain your sister's favorite sweatshirt, she might be mad at you—especially if you didn't ask before borrowing it.
You can stain your favorite white pants when you're eating strawberries, or stain the table top you're making by painting it with a tinted liquid. The resulting stain in both cases is visible. Another kind of stain is a figurative mark or blemish, like the stain on your school records from that difficult semester when you never did your homework. Stain comes, via the Middle English disteynen, or "discolor," from the Latin tingere, "to color or dye."