In A Dictionary of the English Language (1755), the English lexicographer Samuel Johnson (1709-84) thus defined bonfire:

[from bon, good, French, and fire.] A fire made for some publick cause of triumph or exultation.

In support of this etymology, bonfire in several languages is, literally, fire of joy. For example:

– French feu de joie
– Italian fuoco d’allegrezza
– German Freudefeuer
– Dutch vreugdevuur.

But Samuel Johnson was mistaken: bonfire originally denoted a great fire in which bones were burnt in the open air. This is confirmed by the earliest known mention of the word, in the English-Latin wordbook Catholicon Anglicum (1483) (Latin os, ossis, is the origin of, for example, French os and Italian osso, meaning bone):

(1881 edition)
a Bane; os, ossiculum, ossillum; osseus participium.
a Banefyre; ignisossium [= a bone-fire].

The practice seems to have come down from heathen times, and there were in particular bone-fires on the eve of St John the Baptist’s feast, as Christianity conveniently placed this feast on 24^th June, close to the summer solstice (French has feu de la Saint-Jean, and German Johannisfeuer). In The festyuall, a collection of homilies for the festivals of the liturgical year, the Augustinian Canon Regular John Mirk (floruit 1403?) wrote:

(1508 edition)
In worshyppe of saynte Iohan the people waked at home. & made .iii. maner of fyres. One was clene bones and noo woode / & that is called  a bone fyre  A nother is clene wode & no bones. & that is called a wode fyre / for peple to syt & wake therby. The .iii. is made of wode and bones. and it is called saynt Iohannis fyre.

And the Ordinary of the Company of Cooks at Newcastle upon Tyne, dated 1575, contains the following clause:

And alsoe that the said Felloship of Cookes shall yearelie of theire owne cost and charge mainteigne and keep the Bone-fires, according to the auntient custome of the said towne on the Sand-hill; that is to say, one Bone-fire on the Even of the Feast of the Nativitie of St. John Bastist, commonly called Midsomer Even, and the other on the Even of the Feast of St. Peter the Apostle, if it shall please the Maior and Aldermen of the said towne for the time being to have the same Bone-fires.

In 1715, the bookseller and antiquary John Bagford (1650?-1716) wrote:

I have heard of another Custom that is practised in some Parts of Lincolnshire, where, on some peculiar Nights, they make great Fires in the publick Streets of their Towns with Bones of Oxen, Sheep, &c. which are heaped together for some time before. I am apt to believe this Custom was continued in memory of burning their Dead, and that from hence came the original of Bonefires. Which reminds me of what is mentioned in the Office of the Dead in our Liturgy, where these words, (from the ancient Custom of burning the Body,) ‘Ashes to Ashes’, ‘Dust to Dust’, &c. are still retained.
(published in Joannis Lelandi antiquarii de rebus britannicis collectanea – 1770 edition)

The etymological spelling bone-fire (Scottish bane-fire) was common until 1760, though bonfire was also in use from the 16^th century and became more common as the original sense was forgotten. The memory of the original sense was retained longer in Scotland with the spelling banefyre, bane being a spelling of bone which was long common in Scotland.

The following citations show how the spelling shifted from bonefire to bonfire, and the sense from fire of bones to fire of joy:

– from Tamburlaine the Great (1590), by the English playwright and poet Christopher Marlowe (1564-93):

Now will the Christian miscreants be glad,
Ringing with ioy their superstitious belles:
And making bonfires for my ouerthrow.
But ere I die those foule Idolaters
Shall make me bonfires with their filthy bones,
For though the glorie of this day be lost.

– from The Church History of Britain from the Birth of Jesus Christ, until the year MDCXLVIII (1655), by the Church of England clergyman Thomas Fuller (1608-61):

Both parties in gratitude to God would in a bonefire of their generall joy, have burnt this unhappy bone of dissention cast betwixt them.

Both authors knew that bon(e)fires ought to burn bones and punned on the two meanings of the word.

One of the meanings of bonfire is a fire for immolation, particularly a fire in which heretics, bibles or proscribed books were burnt. For example, in The Martyr’d Souldier, published in 1638, the playwright Henry Shirley (died 1627) wrote:

Methinks Christians make the bravest Bonefires of any people in the Vniverse.

The Fifth of November bonfires, lit in memory of the Gunpowder Plot (the plot to blow up the Houses of Parliament on 5^th November 1605, while the King and Lords and Commons were assembled there) combine various senses of the word bonfire:
– a fire for immolation,
– a fire in celebration of an event,
– a great blazing fire made for amusement,
– a great fire for burning up rubbish, thorns, weeds, etc.