Yes, this is another of my favorite words. Don't ask.
Turpitude comes from the Latin "turpis" for "base or vile" and "tude", the suffix which makes the adjective into a participle. And that makes this word as clear as mud. We all have a general understanding of what base and vile mean, but now its important to know what these words really mean. So, for the meta level we have vile, which comes from the Latin "vilis" for "of little worth, base or cheap" but which has come to mean things associated with the poor of the Dark Ages (wretched, filthy, repulsive, etc.) and base, which comes from the Latin "basus" for physically low or short, and which has come to have a similar connotation as with those of low birth (not refined, worthless, morally low). Ok, so back to turpitude. While base and vile etymologically have no origin in depravity, the ongoing association with the individual who were literally base or vile tended to create that connotation and it stuck, not just to base and vile, but by extension to turpitude. The irony is that as now the moral depravity is implied, turpitude is usually paired with moral in an almost redundant fashion. Moral turpitude, as if there is another kind of depravity. Could we really fathom political turpitude? isn't that really moral/ethical anyway. economic turpitude? that just sounds implausible. But ultimately, this is just another connotative association from something of little worth to people without money to what people without money may be forced to do by society a millenium ago to just that conduct generally and finally to a moral issue. Naturally.
As usual, turpitude applies to people and the activities of people almost exclusively. It's kinda hard for my cat to engage in an immoral act, but I suppose it is possible. Let's not go there either. So, the Pennsylvania school authorities maintained the right to fire a teacher for moral turpitude. This was how it actually expressed its right, amusingly for an educational system using the redundancy, and it is still just as vague now as it was then. Adultery, while still on the books as an offense, is no longer actionable for divorce as an independent cause, and therefore, socially, may no longer be considered turpitude. True, although esoteric. Miscreant youths loitering in the mall for shops to close in order to mug the last customers engage in a unique form of turpitude. It is turpitude, however, to misuse a word? Depend of how egregious the misuse. Misusing well and good? probably. Failing to use the subjunctive tense? definitely!