The Real Madrid 'keeper didn't hold back, after receiving criticism from the crowd on Saturday.
As far as Real Madrid are concerned, the title race is practically over, following the 2-2 draw against Valencia on Saturday.
Saying that Los Blancos fans are disappointed and frustrated, would be putting it mildly, as they went looking for scapegoats, as to why their side won't be winning La Liga this season.
The easiest of targets appeared to be captain and goalkeeper, Iker Casillas, who bore the brunt of their annoyance.
Whistles were directed towards the 33-year-old when Valencia scored their first. Paco Alcacer with the finish, Casillas getting a touch, but unable to stop the ball heading into the back of the net.
Worse was to come for Casillas when Javi Fuego netted the Valencia second. The whistles were even louder and continued, every time the Real Madrid goalkeeper made contact with the ball.
Unable to contain his own frustration, at the fickle nature of the fans behind his goal, at a goal kick, Casillas responded angrily. According to radio network Cadena COPE, along with a dismissive gesture, he shouted back to them:
"That's enough already... go stick it up your arse!"
Ever demanding the very best from their stars, groups of fans in the Bernabeu certainly don't hold back with their criticism, singling out players for whistles.
Cristiano Ronaldo has already shown his reaction this season, but thus far, Casillas has been more restrained, even though he's frequently been made a scapegoat when goals have been conceded.
Amidst the criticism from the crowd, there's already been plenty of speculation throughout the 2014-15 campaign, that this could be the final season for Casillas at Real Madrid.
His angry and frustrated response towards the fans, might suggest that he's now ready to venture away from the Bernabeu and the pressures of Real Madrid, for a new opportunity elsewhere.
Arsenal have remained strongly linked towards a move for the twice World Cup winner, with Spanish pundit Josep Pedrerol of TV show, 'Jugones', recently claiming that the Emirates will be where Casillas heads this summer.