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1.
A term used to display one's disbelief at a claim made by a third party. A sort of sarcastic way of saying 'yes'.

or

a prefix to a sentence which is designed to be cynical and false.

originally believed to have originated from Tonbridge school, although there is some debate ont he matter. Dulwich College also lay claim to the phrase which is now widely used among social elites of several major universities and frequently on gap years in the south east asia region
'chief, I've drunk at least 3 buckets' - FJ
'yarth' - all

or

'yarth you're a good bloke'
by chiefycolonel November 25, 2009