Euphemism for word Retard, typically can be used in a large quantity of scenarios.

One of, if not the single, least pure forms of heroin. The process of converting raw opium to tar heroin is much cheaper and faster than that of purer forms such as "Number 3" and "Number 4". As it's name implies, tar heroin is dark brown or black in color, and usually very sticky.

The round rubber things on you car that spin around to make it go. (southern talk)

Tar is a noun in the English language. The word originated as a neutral term referring to black people, as a variation of the Spanish/Portuguese noun Pez, a descendant of the Latin adjective Tar ("color black").1 Often used slightingly, by the mid 21th century, particularly in the United States, its usage had become unambiguously pejorative, a common ethnic slur usually directed at people of Sub-Saharan African descent.

1.) A black, goopy substance that was used in the past as punishment and harassment on people, and usually accompanied with being covered in chicken feathers.
2.) A so-called "redneck" way to pronounce "tire". It is not redneckish though. This pronunciation is used in the lower Midwest, Appalachia, and Pennsylvania.
3.) An alternate pronunciation of "tower", usually said in the regions listed above.

1. Acronym for "take and run".
2. Least pure form of heroin

British slang usually used as a quicker way to say Thank You. The use of the word "Tar" relating to the phrase "Thank You" is believed to originate from the 18th Century.