A jumbuck is a male sheep. You may be familiar with the term as a jumbuck features in Andrew Barton Paterson’s poem, “Waltzing Matilda”.
Banjo Paterson’s poem, written in 1895, is considered a bush ballad as well as Australia’s unofficial national anthem.
The title was Australian slang for travelling on foot (waltzing) with one’s belongings in a “matilda” (swag) slung over one’s back.The song narrates the story of an itinerant worker, or “swagman“, making a drink of billy tea at a bush camp and capturing a stray jumbuck (sheep) to eat. When the jumbuck’s owner, a squatter (grazier), and three troopers (mounted policemen) pursue the swagman for theft, he declares “You’ll never catch me alive!” and commits suicide by drowning himself in a nearby billabong (watering hole), after which his ghost haunts the site.

North Gregory Hotel, Winton, where Banjo wrote Waltzing Matilda
Once a jolly swagman camped by a billabong
Under the shade of a coolibah tree,
And he sang as he watched and waited till his “Billy” boiled,[51]
“You’ll come a-waltzing Matilda, with me.”
Chorus:
Waltzing Matilda, waltzing Matilda,
You’ll come a-waltzing Matilda, with me,
And he sang as he watched and waited till his “Billy” boiled,[a]
“You’ll come a-waltzing Matilda, with me.”
Down came a jumbuck to drink at that billabong,
Up jumped the swagman and grabbed him with glee,
And he sang as he shoved[b] that jumbuck in his tucker bag,
“You’ll come a-waltzing Matilda, with me.”
(Chorus)
Up rode the squatter, mounted on his thoroughbred.
Down came the troopers, one, two, and three.
“Whose is that jumbuck[c] you’ve got in your tucker bag?
You’ll come a-waltzing Matilda, with me.”
(Chorus)
Up jumped the swagman and sprang into the billabong.
“You’ll never catch me alive!” said he
And his ghost may be heard as you pass by that billabong:
“You’ll come a-waltzing Matilda, with me.”
– A B Paterson

Demystifying Australian Language.