It means "the making of false and defamatory statements about someone in order to damage their reputation; slander."
Francis said that until he sees proof that Bishop Juan Barros was complicit in covering up the sex crimes of the Reverend Fernando Karadima, such accusations against Barros are “all Calumny”.
The pope’s remarks drew shock from Chileans and immediate rebuke from victims and their advocates.They noted the accusers were deemed credible enough by the Vatican that it sentenced Karadima to a lifetime of “penance and prayer” for his crimes in 2011.
A Chilean judge also found the victims to be credible, saying that while she had to drop criminal charges against Karadima because too much time had passed, proof of his crimes wasn’t lacking.
http://www.thejournal.ie/pope-francis-child-sex-abuse-victims-3806439-Jan2018/
Very odd that he would require "proof", surely belief in the judge would do!!!

very severe sentence there too, very cruel thing to do.
And we have already moved the all Ireland final for his visit.....
Here come the bad old days of deference and bowing and scraping, it's going to be painful to step back into the seventies again.