In France, you can wish somebody bon just about anything: bonjour, bon courage, bon app, bonnes vacances, and, my personal favourite, bonne fin de dimanche. But one expression which is often avoided is bonne chance. Not because the French are too rational to believe in luck, but because, as in many other cultures, wishing somebody good luck is thought, even more superstitiously, to bring about bad luck. To avoid doing so, in English we have the dramatic "break a leg" and the Italians wish each other the daringly romantic in bocca al lupo ("into the mouth of the wolf").
So what do the French say?
But luckily for those of us who are too refined to resort to scatological humour at critical moments in our loved ones' lives, there is an alternative.
For, during the Battle of Waterloo, the famous General Cambronne was called upon to surrender himself to the British. The story goes that he replied that he would rather die than do so. When summoned a second time, he simply replied, "Merde!"
The British were apparently so impressed by his strength of character that, instead of killing him they merely took him prisoner.
Cambronne denied this for the rest of his life, but the legend nevertheless took root and Je te dis le mot de Cambronne became a perfectly normal way to wish somebody luck.