E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Ignis ossium. The Athenum shows that the word means a fire made of bones; one quotation runs thus, In the worship of St. John, the people made three manner of fires: one was of clean bones and no wood, and that is called a bonefire; another of clean wood and no bones, and that is called a wood-fire and the third is made of wood and bones, and is called St. Johns fire (Quatuor Sermones, 1499). Certainly bone (Scotch, bane) is the more ancient way of spelling the first syllable of the word; but some suggest that bon-fire is really boon-fire.
In some parts of Lincolnshire they make fires in the public streets with bones of oxen, sheep, etc . heaped together hence came the origin of bonfires.Leland, 1552.
Whatever the origin of the word, it has long been uséd to signify either a beacon fire, or a boon fire, i.e. a fire expressive of joy. We often find the word spelt bane-fire, where bane may mean bone or beacon. Welsh ban, lofty; allied to the Norwegian baun, a beacon or cresset.