The word derives from "bone fire", and as early as the 15th century where references are to the bone fires which referred to funeral pyres.  During the 16th century and up to the middle 18th century, the term "bone fire" sometimes also referred to the fires in which those who were considered religious heretics were burned along with their heretical books.  The term was changed in spelling to bonfire, and also gradually in meaning from the mid 18th century to the current rather harmless (and often fun) tradition of huges fires of celebration at beach parties, at Halloween, and in the fall at harvest time to celebrate.  Apparently, the earliest known instance of the derivation of the word occurred in 1483, as in 
"ban fyre ignis ossium" used in Catholicon Anglicis. 

 Hi Yergamon, thanks for responding to my submission on a recent debate thread. Sure, I'll be chasing you for information/advice.  I've been watching you, along with Pencil, Nanny, etc etc. Have found stuff live the above facinating. Cheers keznoa.