Ballast

[Sarah]

Any heavy substance, as stone, iron, etc., put into the hold tosink a vessel in the water to such a depth as to prevent capsizing.

[Sarah]

Ballast is a friendly reference to; a mate, sidekick, wife, bitch, girlfriend, boyfriend, trick, pal, etc. but especially the person who rides [shotgun] with them in their car.
Used by Eddie Haskel to describe Lumpy in an episode of Leave

[Sarah]

A derogatory name given to a person who is withholding a comment/skill/resource in order to purposefully change the atmosphere in which he is residing by then revealing the [sandbagged] item.

[Sarah]

An term of enderment for your girlfriend/wife etc

[Sarah]

The least attractive female in a group of females. Normally, female friends/aquaintances like to attach ballast to their group to make themselves look better.

[Dave Paprocki]

[Something] that [keeps] a [head] verticle.

[Mark Zuckerberg]

Heavy material, such as gravel, sand, or iron, placed in the bilge of a ship to ensure its stability.