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1.
A five dollar bill. Comes from suffixing fin with ster. Works particularly well in card games like poker or Acey Deucey as a chant or taunt to encourage the player to bet $5.
Oh, that's at least worth a finster

A deuce and a seven? Come on, grow some balls. Fin-ster! Fin-ster! Fin-ster!
by flurffmeister August 29, 2011
 
2.
A nerd, lame, dweeb, pointdexter, or the like.
Dude, you're a total finster.
by 2014_chiguy October 15, 2006