[kog, kawg] 
noun, verb, cogged, cog⋅ging.| 1. | (not in technical use) a gear tooth, formerly esp. one of hardwood or metal, fitted into a slot in a gearwheel of less durable material. |
| 2. | a cogwheel. |
| 3. | a person who plays a minor part in a large organization, activity, etc.: He's just a small cog in the financial department. |
| 4. | (of an electric motor) to move jerkily. |
| 5. | to roll or hammer (an ingot) into a bloom or slab. |
| 6. | slip a cog, to make a blunder; err: One of the clerks must have slipped a cog. |
slip a cog
Also, slip a gear or one's gears. Lose one's ability to reason soundly or make correct judgments, as in She must have slipped a cog or she would never have gone out barefoot in December, or What's the matter with him? Has he slipped his gears? These slangy usages allude to a mechanical failure owing to the cog of a gear or a gear failing to mesh. The first dates from about 1930, the variant from the 1960s.