Green’s Dictionary of Slang

leper n.

a general term of abuse.

1974		(con. 1960s) R. Price Wanderers 74: The Hite brothers were idiots. Scottie, ten years old, was best friends with Dougie Rizzo, [...] Scottie’s brother, Rockhead or Frank, was as old as the Wanderers but was considered a maniac jerk-off, and a leper.
1993		S. Heaney Midnight Verdict 32: The creep, I can see why they think he’s great! A star bucklepper [...] the kind of man I would sweep away.
2001		OnLine Dict. of Playground Sl. 🌐 leper adj. A general insult. Usually in relation to a lack of intelligence/ ability – after someone drops a bollock. ‘Christ – you really screwed that up! What a leper!!’.

In phrases

leper in a sleeping bag (n.)

(Aus.) a sausage roll.

2016

N. Cummins Adventures of the Honey Badger [ebook] The orders [...] usually consisted of a rat coffin or a leper in a sleeping bag (sausage rolls), maggot bag, dog’s eye or mystery bag (pies), dead horse (tomato sauce) and battery acid (cola).