According to Chuck Jones’ autobiography, Chuck Amuck, the inspiration for Daffy Duck’s voice was the tyrannical producer of the Termite Terrace gang, Leon Schlesinger, who was as well-known for his lisp as he was for his lack of anything resembling a sense of humor.
After coming up with Daffy Duck (for Tex Avery’s Porky’s Duck Hunt, 1937), they quickly realized that their zany black duck would need to say more than “Hoo hoo!” to remain funny for very long. They asked ace voice actor Mel Blanc (the voice of almost all of the Looney Tunes characters) if he could imitate Schlesinger’s lisp, and he broke up the gang with his dead-on impression.
As they were finishing the cartoon, far too late to turn back, it suddenly dawned on the gang that Schlesinger was going to have to view and approve this cartoon. They were doomed. They literally wrote up letters of resignation to hand in after the inevitable fit that Schlesinger was going to throw.
Schlesinger apparently sat stone-faced at the screening, as the animators squirmed and sweated on their wooden benches. When the lights came up, Leon Schlesinger got up and exclaimed, “Jethuth Chrith, that’s a funny voith! Where did you get that voith?”
And he never knew, to the day of his death, that he inspired that voice.
Jones, Chuck, Chuck Amuck (Avon Books, New York, 1989).