A losing sente is, as it says, a sente play that loses points. Some other local play, normally a gote, but sometimes a sente with a smaller threat, is orthodox play. As a rule, playing a losing sente is wrong, but sometimes it is right.
For instance, you may want to make a losing ko threat. You may also gain more by keeping sente than by losing it and allowing the opponent to take the other side of the losing sente.
There is another meaning for losing sente as in "losing sente to the opponent".
- Losing sente example (not such a good example it seems)