Canossa is a
village in north-central
Italy in the
Appenine Mountains, about eighteen miles away from
Parma. It is mostly known for the events of January
1077, when
Pope Gregory VII, on his way to a meeting in
Augsburg, stayed at the Canossa
castle of Countess Matilda, last of the family who had ruled the
Tuscany area for two centuries.
A year earlier, Gregory had excommunicated the Holy Roman Emperor, Henry IV, as the culmination of several years' conflict over who could appoint church officials; now Henry was in danger of losing the support of his German vassals unless he was reaccepted by the Church. Gregory reportedly holed up at Canossa to protect himself when he heard that Henry was coming to Italy. However, Henry's aim was not to threaten the Pope but actually to do penance -- he stood outside the castle for three days, barefoot in the snow during one of the harshest winters then recorded, and dressed as a penitent. This was seen as more of a personal humiliation for Henry than an admission that the Emperor was subject to the Pope. But Matilda and members of the Pope's entourage persuaded him to see Henry and accept him back into the Church.
However, Henry did not last long before going against the Pope again; he would be excommunicated again in 1080. (Shortly before Gregory's death in exile in 1085,
he withdrew all the censures of excommunication that he had pronounced, except those against Henry and Guibert, the "antipope" consecrated in 1084 with Henry's support.)
Otto von Bismarck would use the phrase "we're not going to Canossa" ("Nach Canossa gehen wir nicht") in his Kulturkampfstruggle between the Church and his German government to mean that he would not submit to the Catholic Church's demands. The phrase "go to Canossa" is also used in many languages ("over 30" languages, according to one source) to mean humilating oneself before a former enemy, or eating humble pie to keep hold of some amount of power or position.
Sources:
http://www.newadvent.org/cathen/03298a.htm
http://www.newadvent.org/cathen/06791c.htm
http://www.bartleby.com/65/ca/Canossa.html
http://wordsmith.org/awad/archives/0203
http://wordsmith.org/words/kulturkampf.html
http://www.historybookshop.com/articles/places/canossa.asp
http://fbc.binghamton.edu/66en.htm
http://www.reappennino.it/REapp/inglese/schedaTerre.txt
http://www.abbc.com/aaargh/engl/hay/bobprefaceEa.html