Short for 'Junior Underprivileged Brat'.
Boarding School lingo for anyone younger than you, but normally reserved for the youngest pupils.
The term originally stems from a poem about 'Jumblies' by Edward Lear, and was intended to be affectionate, but has been corrupted overtime. First used at Quantock School (which has now closed)
Check out that Jub, he'll be crying for his mummy before the night is out!