Audience members were left shocked during filming for the new series of the ITV talent show when the comedian made the remark to talented Doncaster youngster Brook Exley.
Brook, 12, from Scawthorpe, told the judges that his pet rabbit he used for magic tricks had passed away as he took to the stage on the first day of BGT auditions at The Winter Gardens in Blackpool on Tuesday.
However, the Ridgewood School pupil was left visibly shocked after David, 46, made the quip, according to The Sun.
When fellow judge Amanda Holden asked Brook if he had ever pulled a rabbit out of a hat, he told the judges: “I tried to do that once but he kind of snuck out of the hat and I lost him for a day.
“His name was Albert, we don’t have him anymore, he died.”
David replied: “Don’t worry, he’s dog food now” – sparking shocked gasps from the audience.
Brook then wowed the judges with a card trick and Walliams told the contestant he “f***ing loved it.”
And he then claimed Alesha Dixon was drunk as she gave her thoughts on the audition and said Brook had used “powerful armography”.
He told her: “Armography, that’s a new term. She’s been drinking again.”
Brook has already appeared on ITV children's weekend morning show Scrambled to show off his skills - leaving hosts Luke and Sam baffled.
He has also starred in a BBC1 drama Harriet's Army and a touring production of The Full Monty.