In bocca al lupo

In bocca al lupo (; "into the wolf's mouth") is an Italian idiom originally used in opera and theatre to wish a performer good luck prior to a performance. 
The standard response is crepi il lupo! (; "may the wolf die") or, more commonly, simply crepi! ("may it die") .
It has been proposed, for example by animal welfare activists, to instead reply with viva il lupo! (IPA: [viva il lu:po]; “may the wolf live”), but this is not a standard or common reply.
Equivalent to the English actor's idiom "break a leg", the expression reflects a theatrical superstition in which wishing a person "good luck" is considered bad luck. The expression is commonly used in Italy off stage, as superstitions and customs travel through other professions and then into common use, and it can sometimes be heard outside of Italy.
Origin.
Its use originated with hunters wishing each other to be in dangerous situations. The superstitious use of wishing a negative or dangerous situation as a way of wishing good luck is common in other languages. Indeed, the general image of the wolf in the common language, both in Italy and in the Western culture, is that of a dangerous, hungry and violent creature (e.g. "wolf in sheep's clothing", "cry wolf").
Alternative idioms.
An alternative operatic good luck charm is the phrase "toi toi toi", originally an idiom used to ward off a spell or hex, often accompanied by knocking on wood, and onomatopoeic spitting (or imitating the sound of spitting). Amongst English actors "break a leg" is the usual phrase, while for professional dancers the traditional saying is "merde", from French for "shit". In Spanish and Portuguese, the phrase is respectively "mucha mierda" and "muita merda", or "lots of shit".