If you are patronizing, you tend to speak down to others, acting as though you are smarter, classier, or just plain better than anyone else. For the record, your snobby attitude is not impressing anyone.
A "patron" is someone who subsidizes or finances another person or organization — usually an artist or charitable institution. So the word patronizing stems from this idea of someone bestowing money or help on another. Today, the sense of condescension is strongly attached to the adjective, while the word "patron" is still a positive description of someone who gives.