What does "grit" mean?

Additional senses

1. 2.pl. Grain, esp. oats or wheat, hulled and coarsely ground; in high milling, fragments of cracked wheat smaller than groats.
2. 3.(Geol.) A hard, coarse-grained siliceous sandstone; as, millstone grit; -- called also gritrock and gritstone. The name is also applied to a finer sharp-grained sandstone; as, grindstone grit.
3. 4.Structure, as adapted to grind or sharpen; as, a hone of good grit.
4. 5.Firmness of mind; invincible spirit; unyielding courage; fortitude. C. Reade. E. P. Whipple.
5. 6.To give forth a grating sound, as sand under the feet; to grate; to grind. The sanded floor that grits beneath the tread. Goldsmith.
6. 7.To grind; to rub harshly together; to grate; as, to grit the teeth. [Collog.]