a noun or adjective indicating anything a high level of knowledge in some field (i.e. fashion); since The Pirates of the Caribbean came out, it has often been equated with "understand"
Regarding the etymology of the word, it is most likely another anglified French word, from the original "savoir," which means "to know." Like parley (also used in "Pirates"), which means "to speak," it was probably absorbed into the English vocabulary back when French was more of a global language.
He's very computer-savvy.
She's such a savvy dresser.