meaning - How does the "be-" prefix change the words to which it is applied? How did it come about?

The be- forefast (prefix), for me, is one of the funnest to play with. It often changes the meaning of the root word in an unforetellable way.

A couple of clarifications. The be- mostly acts as an intensifier. For byspel, the verb to head means to lop off the upper part (OED verb, #5): the willow is headed every three or four years. The verb to behead is merely intensifying this meaning but is normally only used with executions.

The trick is often to know which meaning is being intensified.

To reword what Alain said and put it in a different way, it can take a noun or adjective and make it into a verb. Thus if I want a verb meaning to color something purple, it would be bepurple.

To besot is a byspel of turning a noun into a verb. It means to be a fool, act a fool, be made of a fool of thus besotted (also besotten) means drunk, stupefied, or strongly infatuated with (OED): he became besotted with a local barmaid. From be- + sot (from OE sott — foolish (adj), fool (n)). Oddly enuff, the verb in ME was assoten, assotten.

Bename can mean to name, declare, or to nominate (nominate is from Latin nominare, to name).

It also turns an intransitiv verb into a transitiv one. The archaic bedive means to submerge, dip, or drown something. Thus bedoven means drenched, drowned. I was bedoven with sweat.

It's a fun forefast!