" Always listen to your elders, be they wise or moronic. It's a form of the subjunctive that I think survives in this fixed expression; a grammarian would be able to explain this better than I can.
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MichalS'be that'be is the present subjunctive of (what else?) be. In this context be X means whether X is/are in modern English -- whether X be in older English. Thus,
DelmobileFee, fi, fo, fum,
I smell the blood of an Englishman!
Jesus wept !
When I saw it, I knew right away it had a familiar ring to it, then it dawned on me - this verse bears a striking resemblance to the first verse of a song by Eminem (from his new album - Relapse)
It goes
Fee, fi, fo, fum,