The dictionary defines "sass" as impudence or cheek, but it's really much more than that. Sass is about an attitude that makes people laugh in the face of danger, that speaks truth to power, and that doesn't let its fears control it.
Being sassy means dressing the way you want to dress, whether it's someone else's idea of fashion or not. It means changing your career or job if you're realizing that your current course simply doesn't suit who you intend to become. It means not being afraid to reinvent yourself. Most of all, it means carrying yourself in such a way as to tell the world that you do not take crap from anyone. There are certainly other ways to take control of your life, but sass is a great place to start.
A sassy person is usually very witty, meaning some people warm towards them quickly. However, others may take exception to sass, seeing it as inappropriate or as somehow disrupting the natural order of things - which of course, is often exactly the point.
The important thing about sass is that while you may use it to lightly mock a friend, most of all it is about punching up, never down. It punctures pomposity and helps people see the absurdity of a situation. You can learn it a little, but usually you're born with it. So click on through and let's see just how much sass you've got!