From Old English 'bān,' distinctly Germanic with no clear PIE source — and 'bonfire' was originally 'bone-fire,' fueled by animal bones.
Any of the pieces of hard whitish tissue making up the skeleton in humans and other vertebrates.
From Old English 'bān' (bone, tusk), from Proto-Germanic *bainą (bone), of uncertain PIE origin. Some scholars connect it to PIE *bʰeh₂- (to shine, to be white), referring to the whiteness of bone. The word is exclusively Germanic — Latin used 'os' (bone, from PIE *h₃ésth₁), Greek used 'ostéon' (same root), and these gave us 'ossify' and 'osteopath' respectively. Key roots: *bainą (Proto-Germanic: "bone (uncertain further origin)").
'Bonfire' was originally 'bone-fire' — a fire fueled by bones. In medieval Britain, bones of slaughtered animals were burned in large outdoor fires, especially on midsummer festivals. The spelling shifted from 'bonefire' to 'bonfire' as the original meaning was forgotten. And 'trombone' means 'big trumpet' in Italian — the 'bone' is a coincidence.