present plural indicative of be (q.v.), from Old English earun (Mercian), aron (Northumbrian), from Proto-Germanic *ar-, probably a variant of PIE *es- "to be" (see am). Also from Old Norse cognates.
In 17c. it began to replace be, ben as first person plural present indicative in standard English. The only non-dialectal survival of be in this sense is the powers that be. But in southwest England, we be (in Devonshire us be) remains non-standard idiom as a contradictory positive ("You people aren't speaking correct English." "Oh, yes we be!"), and we be has reappeared in African-American vernacular.