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1.
FRN
Fucking Railroad Nut; used by railroad employees to describe those who watch and/or photograph trains. Usage has recently fallen off with "foamer" being used in many parts of the country instead
We saw a ton of FRNs stopped by the side of the tracks when the steamer was coming.
by Schnauf March 07, 2006
 
2.
frn
Federal Reserve Note, a paper printed as to resemble official money, as opposed to gold or silver coin or a United States Note, ostensibly backed by gold and/or silver.
How many FRNs does that cost?
by Peter314 January 17, 2004
 
3.
frn
Stands for "Face right now".
"Look at this picture of a cute cat. IS IT NOT CUTE?"
"frn -> :D"

"FML, we broke up. frn -> :C"
by Thisusernamewasalreadytaken August 21, 2011