In
Psychological terms: to be of many moods and/or to change moods rapidly. Often reported as a
symptom (clinical indicator) for
Bipolar disorder.
Labile in this sense does not mean you are a "little moody." It means you are a ton moody in less than a minute. It means going from sad and tearful to giggly and bouncy in a short (less than five minute) period, without any chemical intervention.
It could be demonstrated by someone who would tell you in a serious tone about the recent death of a pet, then at the end of the story start telling sick jokes about animals while jumping up and down and acting out the stories while laughing hysterically. These would be the people who call you up and curse you out, hang up the phone, then call you back and see if you want to go see a movie. They are not joking .
Labile means extreme shifts in behavior with no apparent reason and no grasp of how this might affect others (oblivious to their surroundings). People who are this moody don't get why people are looking at them "that way."