An extremely massive person.
"Her? She's a jumbo freakin' jet!"
A man with a large penis and massive testicles
A jumbo Jet refers to the fact that the owner of the afore-said jumbo sized tackle is able to force a correspondingly large amount of semen from it during ejaculation
Fuckin ell that were a feller I had last night let me tell ya! That were a proper fuckin jumbo jet I ad flying into my terminal 5 last night