Keane used that word to describe the keeper in the build-up to England's game against Poland.
Hart went on to make the mistake that led to the home side's equaliser in the 1-1 draw in Warsaw.
But City manager Mancini insisted: "If you want to be a good keeper, you should be cocky sometimes.
"Every goalkeeper is like this. It is important because it's different to another position. If you want to be a keeper, you should be cocky.
"Last week, Wayne Rooney said on Twitter that Joe is the best keeper in the world, but today he is cocky.
"It is my opinion that Joe is the best keeper in England and one of the best in Europe."