Rakish, irresistible; or rather that care-for-nobody, jaunty, daring behaviour which has won for many of our regiments the term as a compliment. It is also applied metaphorically to some inanimate things, as “saucy waves,” which dare attack the very moon; the “saucy world,” which dares defy the very gods; the
“saucy mountains,” “winds,” “wit,” and so on.
“But still the little petrel was saucy as the waves.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Saucy from Infoplease: