It is an adjective for something "big, of enormous size", from the name of an Abyssinian elephant captured in the mid-19th century and exhibited in circuses and fairs in Europe and the US. There are no records that explain why they gave it that name, perhaps it has its origin in the Swahili greeting yambo, which is how Jumbo sounds in English, although for some reason in many Spanish-speaking countries we say it 'yumbo'.
