The common word Jumbo didn't start out meaning 'big' or 'huge' as in "jumbo jet". It actually has it's origins in the 1860's from a orphan African elephant that was put into the London zoo. He was named "Jumbo", not because he was large, although he actually was very large, but because he was named "hello", in Swahili. The name "Jumbo" was a mispronunciation of "Jambo".
Jumbo was large even for an African elephant. At the time, no one in London had ever even seen an African elephant, so even a normal one would have been large. However, Jumbo was 3 meters tall by time he was 21, which is more than 20% taller than others his age.
He was very popular and well known in England, but in 1882, PT Barnum bought Jumbo for 2000 pounds. He was shipped to the US with his trainer, who he refused to go without. Jumbo arrived in New York with much fanfare and PT Barnum exaggerated his already enormous size saying he was the largest land animal in the world.
All the hype and the actual large size of Jumbo made the word synonymous with the word huge. Sadly, Jumbo died when he was only 24, after being hit by a freight train in St. Thomas, Ontario, while being loaded onto his car.
A side note: After learning this, I then realized where the Disney elephant "Dumbo" got his name and I find that kind of funny.
