Jumbo was a bull African elephant, possibly the largest elephant ever held in captivity. He was a resident of the London Zoo, famous for giving rides to children. Zoo officials became worried about the danger the 20 year old, 12 foot tall elephant might pose, and he was sold in 1882, to P.T. Barnum. Jumbo became a star in the Barnum and Bailey circus.
Jumbo was killed in St. Thomas, Ontario, Canada in 1885 when he lost an argument with a locomotive of the Grand Trunk Railroad. The train derailed. Barnum's circus carried on after the loss, exhibiting Jumbo's preserved hide (stretched over a wooden frame) and skeleton. The skeleton was donated to the American Museum of Natural History in New York, the hide was indeed donated to Tufts University, and suffered the fate described in the node Jumbo.
Jumbo's name came to mean anything huge, gigantic, or an exceptionally large specimen. Even today, most children associate the name "Jumbo" with elephants.
This word was coined around 1880, which may (or may not) explain why Webby does not know it.
From several sources including:
http://www.barnum-museum.org/html/jumbo.html