behind (adv.) Old English behindan "behind, after,"
from bi "by" + hindan "from behind
" (see hind (adj.)).
hindan already meant "from behind", and It doesn't make sense to say: by from behind
.
So why was bi "by" added as a prefix?
How did bi "by" contribute to 'behind'?
Unlike Etymonline above, OED doesn't pinpoint the matching definition of the prefix be-.