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To stick ones finger up an arse, either your own or someone elses, and to then smear this finger and its contents under the nose of the victim. This gives them a moustache of shit - or a ratty tash.

Victims can be lulled into the act by asking them "What does this smell like to you?"
When Sweetman was younger he gave his entire family a ratty tash on Christmas Day - the mucky bugger - using the old "What does this smell like to you?"
by SwiftSix June 20, 2005