Like the term Freakbeat and Black Rock, Bonehead is a recent term used to describe a specific SOUND from the mid '60s/early '70s. As quoted by the compilers of the Bonehead Cruncher/Crusher compilation series Bonehead is the sound of "garage punk transitioning to heavy rock." So, basically it's long hair groups who play loud and HEAVY, and stupid, riffs. Um, think groups influenced by the Cream, the Experience, and Blue Cheer, but real stupid. Yeah...stupid; these groups ain't boot stompin' glam/tarted up junkshop boogie, smart enough to write pop, or really good enough to play snooty prog. Ah, BONEHEADED! However, as I understand it these kinda bands, if they rated in like, 1970, were called "BIKER bands." Right.
Honestly, I love the shit outta this kinda shit, but I'm not sure if I agree with Bonehead being used for transitional term. A retro applied term like Freakbeat is really specific to a time/sound, but stretching out Bonehead as garage-to-heavy evolution that took place over the course of ten years, well...that didn't happen. Oof, whatever...there you go.