Turpitude means deeply wicked, shameful behaviour — not a small mistake or an unkind moment, but the kind of wrongdoing that makes people think badly of a person's whole character. It is a very formal word, and you should know that in real English it almost never travels alone: it lives inside the fixed phrase moral turpitude, which is legal language. American immigration law and professional licence rules use it to name a class of crimes serious enough to get someone deported, or struck off, or refused a visa. Courts have argued for a hundred years about exactly which crimes count, which tells you the word is vaguer than it sounds. Outside law, a writer may use it for effect, and then it usually carries a small smile — it is a heavy, old-fashioned word, so choosing it can sound grand or joking rather than serious. In ordinary conversation nobody says it.

（極正式）道德敗壞、卑劣行為。幾乎只出現在法律片語 moral turpitude（道德敗壞罪行），常見於美國移民法與專業執照規定。