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1.
Sparse moustache that resembles a rat on the upper lip
14 year old Johnny was proud of his facial hair, oblivious to the fact he was sporting a rat tash
by GRANKOR July 29, 2011
 
2.
A male (or in some unfortunate occassions a female) with very poor facial hair resting on ones upper lip resembling that of a rats whiskers.
He're Gibby ya wee Rat Tash havin bastard, get the beers in!!
by ChrQ January 20, 2009