1853, of uncertain origin, probably related to spiff "well-dressed man." Spiffing "excellent" was very popular in 1870s slang. Uncertain relationship to spiff (n.) "percentage allowed by drapers to their young men when they effect sale of old fashioned or undesirable stock" (1859), or to spiflicate "confound, overcome completely," a cant word from 1749 preserved in American English slang spiflicated "drunk," first recorded 1906 in O.Henry.
Elegant; excellent; snazzy: They wear spiffy red-and-gold scarves/ New Model Buggy for Amish Is Spiffy (1853+)
Well; elegantly: They don't translate so spiffy (1937+)