Behead is from Old English, the head part is still sufficiently understandable, but be- is no longer a productive prefix. It is so old that it appears in words with a wide range of meanings. It originally meant something like ‘about’, which is how we get senses like before (‘about the fore’), behind, and beyond. This broadened ‘all around’ in words like besplatter, then became an intensifier in bemuddle. Another sense was deprivation, such as bereave; reave meaning ‘to steal’, now retained euphemistically for death. This is also the sense that gave us behead, but be- can no longer be used to refer to the deprivation of other body parts.
This piece originally featured in The Big Issue in Australia. By Lingo is found on the puzzles page in every edition. Every couple of months I drag out at old favourite to share here on Superlinguo. If you’re in Australia you can pick up a copy of The Big Issue from one of the street vendors every fortnight! For those who aren’t in Australia, ‘behead’ featured in edition #495 in 2015.