Kikki's Workshop Q&A: Let's ask Kikki ! Kikki's Workshop
Where did the name Jumbo come from ?
Why do we call hydraulic excavators Jumbo ?
Jumbo was a product name for hydraulic excavator that was made by a French company. Jumbo was imported by Japan during the 1950's and the word Jumbo was used to call hydraulic excavator ever since in Japan.

Additionally, "backhoe" and "power shovel" are sometimes used, though "hydraulic excavator" is becoming standard nowadays.