It's the song that goes like;
Boney was a warrior
Away, a- yah!
A warrior and a terrier
Jean Francois!
Boney fought the Russians
Away, a- yah!
The Russians and the Prussians.
Jean Francois!
I couldn't seem to find anything through google. It seems utterly irrelevant to the rest of the song. Was he an actual person or just a random French name chosen for the sake of rhyme?
Jean François likely came from the French shanty Jean François de Nantes, which had a similar tune. There are similarities in the lyrics of the two songs: the French song has oué, oué, oué instead of away ah-yah, and at least one version of Jean François de Nantes uses the word terrien-landlubber-which mirrors the use of terrier.
Source: Hugill, Shanties from the Seven Seas