What's turpitude, as in moral turpitude"?"

Turpitude is baseness (lack of the higher qualities of the mind or spirit), vileness, or depravity.

In Dr. Jekyll and Mr. Hyde, Dr. Lanyon witnesses the transformation of the creepy-scary Edward Hyde into the good and excellent Henry Jekyll. Observing the metamorphosis, Lanyon can only scream, "Oh God . . . Oh God" again and again as he watches Hyde become Jekyll. When the transformation is complete, the two men talk for an hour.

Dr. Lanyon later writes:

My life is shaken to its roots; sleep has left me; the deadliest terror sits by me at all hours of the day and night . . . . As for the moral turpitude [Hyde] unveiled to me, even with tears of penitence, I cannot, even in memory, dwell on it without a start of horror.

For Lanyon, the actual horror of the discovery that Jekyll and Hyde are the same person lies not in the transformation itself but in the full realization about the nature of evil found in all men: Man does indeed possess an actual evil nature that can be isolated from his dual-natured self.