To grind them.

1

1797.  Mr. Adams, who, gritting his teeth, said, “damn ’em, damn ’em, damn ’em, you see that an elective government will not do.”—Thomas Jefferson, ‘The Anas,’ Dec. 26.

2

1823.  The harmony arising from the filing of a saw, or the gritting of teeth.Mass. Spy, April 30.

3

1847.  To my utter astonishment, however, instead of demolishing me upon the spot, he turned sharply round, and gritting his teeth with a noise like a millstone, walked abruptly out of the room.—Yale Lit. Mag., xii. 200 (March).

4

1848.  

        Just conceive how much harder your teeth you ’d have gritted,
An ’t were not for the dulness I ’ve kindly omitted.
J. R. Lowell, ‘A Fable for Critics’ (N.E.D.)    

5

1850.  Instead of fainting, Lucy uttered a stifled shriek, and gritting her teeth dragged me into the house, the door of which stood invitingly open.—H. C. Lewis (‘Madison Tensas’), ‘Odd Leaves,’ p. 136 (Phila.).

6

1852.  She grits her teeth (!) even now whenever she thinks of him!—Knick. Mag., xl. 178 (Aug.).

7

1878.  He could no longer stand behind the door and grit his teeth when the troubles of polygamy pressed upon him.—J. H. Beadle, ‘Western Wilds,’ p. 364.

8