Contraction of the phrase: Thrift Store Pimpin'. It refers to a case in which a person makes unfashionable clothing purchases from thrift stores which are neither stylish nor up to date. When a person wears said clothing item to a party or social gathering and acts as if they are the best dressed, most stylish person there they are said to be Thrimpin'.
"Dude, Check out the guy in the brown velvet track suit, rockin' aviators and the pump up kicks. Somebody's definetely big thrimpin' tonight."