There are a couple of different answers which kind of fit but I think the answer is
Brushing your teeth
When speaking of doing this everyday as a daily routine, this is a civilized thing to do.
Brushing one's teeth (preferably 2-3 times a day) is part of a healthy daily routine
When speaking of doing this only when something's wrong, this is a nasty thing to do.
If something is wrong with your teeth, then it usually requires intervention by a dentist. This is both nasty with respect to the pain caused and also from a hygienic perspective.