Entry in progress—B.P.
 
“Nifty” is used in the financial terms “Nifty Fifty” and “Nifty Nine.”
 
“Nifty” is said in Wikipedia to mean “National Index for Fifty,” but those words don’t appear in either The New York Times or The Wall Street Journal.
   
Merriam-Webster Dictionary
nifty
adjective nif·ty \ˈnif-tē\
: very good, useful, or attractive
Origin of NIFTY
origin unknown
First Known Use: 1865
 
(Oxford English Dictionary)
nifty, adj.
Etymology:  Origin unknown.
The explanation in quot. 1865 at sense 1 is unlikely, although the influence of the syllable -nif- , in e.g. magnificent adj., is conceivable. Perhaps compare also natty adj.1 and thrifty adj.
Chiefly U.S. Smart, stylish; attractive; of good quality.
1865   B. Harte in Californian 3 June 9/3   Smart, you bet your life ‘twas that! Nifty! Short for magnificat.
1882   ‘M. Twain’ Innocents at Home ii,  He was always nifty himself, and so you bet his funeral ain’t going to be no slouch.
   
California Digital Newspaper Collection
12 May 1863, Marysville (CA) Daily Appeal, pg. 1, col. 7:
When he moopes about the street, and even refuses his accustomed whisky, at news of Federal victories, but goes forth radiantly when traitors succeed, he is a nifty Unionist. (...)—Nevada Transcript.
 
California Digita Newspaper COllection
14 July 1864, Daily Alta California (San Francisco, CA), “Court Proceedings,” pg. 1, col. 5:
... when told that he must ante fifty, in greenbacks gay and nifty, or be drafted for the chain-gang for carrying a pistol.
 
California Digital Newspaper Collection
16 December 1864, Sacramento (CA) Daily Union, “Letter from San Francisco,” pg. 5, col. 4:
Horse-races, cock-fights and rat-pits “were his style.” Such things were “nifty,” and everything else was not!