I sent the following to Italian relatives: buona fortuna domani contro Olanda. They replied "crepi il lupo." I understand the "lupo" but not the "crepi."

Ok let me try - and then our Italian friends can let me know if it is right! I believe the expression comes from hunting, as in "hope you get so close to the wolf that you can kill it easily" and the other person answering "crepi il lupo" really is saying "may the wolf die." So it is an expression of good luck in a particular context, hunting, that has become a general expression of good luck.

... and the first part of the phrase to which "crepi il lupo" is the customary response is: "In bocca al lupo." It's used extensively in the opera world as the equivalent of "break a leg." !

Often shortened colloquially to:
"In bocca" with the response "crepi".

One of my teachers at the Universita` per Stranieri said she used to love the expression... until people began to say "in bocca al professore" with the response "crepi il professore".