By definition

Slug (n.)

1. Any of various small, snaillike, chiefly terrestrial gastropod mollusks of the genus Limax and related genera, having a slow-moving elongated body with no shell or only a flat rudimentary shell on or under the skin.

2. A small metal disk for use in a vending or gambling machine, especially one used illegally.

3. Physics. The unit of mass that is accelerated at the rate of one foot per second per second when acted on by a force of one pound weight.

4. A unit of mass that sits in front of the television and refuses to be acted on by any force whatsoever, or to act as a force in its own right.