I became familiar with the term Labile while reading the book Guilty by Reason of Insanity. Author M.D. Dorothy Otnow Lewis was describing the psychological state of mind of Johnny Garrett who frequently exhibited free and uncontrolled mood swings or behavioral expressions of his emotions.
At the bottom of page 203, she wrote that murderer Johnny had “the impulsiveness, labile moods, poor judgement, and paranoia of a prize fighter who has weathered too many rounds” of boxing. Otnow is also credited with the catchy and accurate phrase: “We have learned that somebody has to be really crazy to be called a schizophrenic by a death row psychiatrist”. The We in previous sentence appears to be Otnow and her husband who spent a lot of time observing those with a predilection for violence at the penitentiary in Huntsville, Texas. Nun Killer Garrett seemed to intrigue her the most as she was developing her reputation as an expert witness in high profile murder cases like the ones involving Mark David Chapman and Arthur Shawcross.
I don’t much like how people like Dr Lewis are part of making the names of cold blooded killers household names to the point stardom. Her fascination with violence is personal tracing back to her somewhat dark childhood.
Anyways, back to the origin or etymology of the term Labile. It comes from Latin word labilis meaning to fall or apt to slip, transient.