Something that's grainy feels rough to the touch, as though it's made of many tiny pieces. A piece of rough sandpaper feels grainy.
Cornmeal is grainy, and your floor will feel grainy under your bare feet after your family returns from the beach with sand on their shoes. Grainy things are gritty, textured with little bumps or grains. You'll be disappointed if your birthday cake has a grainy texture. The earliest, thirteenth-century meaning of grainy was "scarlet dye," a result of its Old French root, which had several meanings including "seed" and "berry." The Latin origin is granum, "seed, grain, or small kernel."