In British slang, "tar" is an abbreviation of "ta," which is a colloquial term for "thank you." The use of "ta" as a casual way of saying "thank you" is believed to have originated in the 18th century and is still commonly used in various parts of the UK today.
Language and slang evolve over time, and "ta" likely emerged as a shortened form of "thank you" for the sake of convenience and informality in spoken language. Over the years, this abbreviation became widely accepted in British culture, particularly in informal or friendly settings.
It's worth noting that slang and colloquialisms can vary between different regions and social groups, so while "tar" or "ta" might be commonly used in some parts of Britain, it might not be as prevalent in others. Additionally, language is constantly changing, so it's possible that new slang terms for "thank you" have emerged since my last knowledge update in 2023.