Cog carries the following dictionary definitions:
1. One of a series of teeth, as on the rim of a wheel or gear, whose engagement transmits successive motive force to a corresponding wheel or gear.
2. A cogwheel.
3. A subordinate member of an organization who performs necessary but usually minor or routine functions.

2nd meaning for Cog:
A meaning of Cog that differs from above is as follows:
1. To cheat, especially at dice. (verb)
2. An instance of cheating; a swindle. (noun)

Do not forget this meaning.

Idioms based on cog:
There are a couple of interesting idioms based on the word cog:
1. Slip a cog: To make a blunder; err
One of the waiters must have slipped a cog.
2.  A cog in the wheel/machine: one part of a large system or organization.
He was just a small cog in the large wheel of organised crime.

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