One theory is that it is a reference to the laurel wreaths given to victors, with the "bacca" being a variation of the Latin word for berry.

L. bacca berry, in allusion to the round shape; or perh. fr.

He then, in a surly way, said to his mother, "I wants some bacca and some beer: that's what I wants; and I'll have it!"

He only wanted bacca, or an old rag of clothes, or a penny, or a bit of meat—bacca first, always; and there was nothing savage about him except his looks.

We now no longer perpetuate their creed, or retain any of their material predilections, except those for beef, beer, "bacca," and bills.