A bonfire today is a friendly thing — a beach gathering, a garden bonfire on a cold autumn evening, a celebration. Its name records something darker. The original Middle English form was banefire, later bonefire: a "fire of bones."
A Burning of Bones
In medieval England, the bonefire had several practical and ritual uses. Butchers piled up the discarded bones of slaughtered animals and burned them — both to dispose of waste and, popularly, to ward off plague and evil spirits. The smoke of burning bone was thought to be cleansing. During outbreaks of pestilence, such fires were lit on a city scale; in religious persecutions, the bones of dead saints (or of executed heretics) were sometimes ritually burned as a final act of denunciation.
From Macabre to Cheerful
By the late 16th century, "bonfire" had broadened to mean any large outdoor fire built for celebration or signal — on saints' days, royal birthdays, and after victories. The word was no longer literal; nobody collected bones for a Guy Fawkes Night fire on 5 November. But the older, grimmer meaning was still close enough to the surface that lexicographers in the 18th century sometimes felt the need to comment on it.
A Folk-Etymology Detour
Some 19th-century writers, embarrassed by the bones, tried to derive "bonfire" from the French bon ("good") — "a good fire." This is folk etymology: a sound-alike rationalisation, not a real history. The actual origin in the Old English bān ("bone") is well documented from the Middle English period, and the older form banefire survives in some Scottish dialects to this day.