Green’s Dictionary of Slang

up your jumper! excl.

also ...gumboots! ...jersey!

(Aus.) a euph. synon. of up your arse! excl.

[UK] in J. Cleary Just Let Me Be in DSUE (1984).
[UK]M.F. Caulfield Black City 44: ‘Flynn, you bitch’s bastard, look what you’ve done?’ ‘Up your jersey,’ said Flynn.
[US]J.E. Macdonnell Jim Brady 15: We snuck alongside and called on ’em to surrender. Not they! Up y’r jumper, f’r ours, they shouted in their own lingo.
[Aus]D. Maitland Breaking Out 6: ‘Up your gumboots,’ he muttered.