What it means:
A big ol' balloon. People ride in it. Mostly just mustachioed villains these days.
What the hell is it supposed to sound like?
What most people know as a blimp is more technically (correctly) known as a dirigible, or a non-rigid airship. The word "blimp" only came about in 1915, when Lt. A.D. Cunningham of the British Royal Navy Air Service decided to strike the side of a royal airship with his thumb, and it made a sound that he interpreted as "blimp." Which was a pretty ballsy move, considering how fragile dirigibles were back in the day.
"Oh my God. Oh Jesus, I only tapped it. I'm so sorry."
Cunningham liked the sound so much that he repeated it incessantly. Inexplicably, his Royal Navy Air buddies were so entertained by Cunningham's insane mutterings that they repeated the story to other officers, instead of drowning him in unsalted water, as British Maritime Law demands.