Something or someone spiffy is fancy or dressed up, like the spiffy red velvet tuxedo you're wearing to your cousin's wedding.
If you're looking for an informal way to compliment someone's outfit, spiffy is the word you need. Your brother may look particularly spiffy on his first day of school, in his brand new sweater and expensive jeans. You could also say he looks dapper, dashing, or snazzy. The adjective spiffy is an American invention, along with the now-obsolete spiff, "well-dressed man."