Note: Hello, TTAG readers! Thanks for the definition suggestions so far. Keep 'em coming!
 
Assault Rifle: A semi-automatic rifle with a plastic stock, which is painted the color black and looks scary. Can be differentiated from other (non-assault) semi-automatic rifles, which are mechanically identical yet are made with wood or painted colors other than black, and do not look scary.

Civil Right: Something that is given to you by the government, and that is paid for by other taxpayers.
 
Fair Share: The portion of the nation's income and wealth that may be objectively apportioned by a government official to any single person based on their deservedness.
 
For the Children: An expression by which one may neutralize or otherwise circumvent any argument, fact, statistic or premise.

Fat Cat: Person whose current income is in a bracket in which it is desirous that tax rates be increased.

Gun: A mechanical device, often clinged to, which spontaneously goes off, killing people.
 
Loophole: Any gun-related feature, device, or accessory, now in production, that is not currently prohibited by law.

Shall Not Be Infringed: This phrase often appears in documents from the late 18th century; it is a typo. Its meaning can be correctly interpreted as either "Shall Be Infringed" or "Shall Be Prohibited".