The definition of sassy is a person who doesn't put you down in a disrespectful way but rather a cheeky way, sassy people are only mean when taken to far or annoyed, so don't piss off a sassy person.
Someone who is full of themselves but in a good way. They're cheeky, lively, smart, saucy, slightly impudent, mouthy, cocky, energetic, loud and extremelytalkative.