The word beetle is known from Anglo-Saxon times: bitula. We find it several times in Shakespeare, as when Isabella talks about "the poor beetle that we tread upon" (Measure for Measure, III.i.82).
But in Old English we find bietel referring to a kind of beating instrument - which by Shakespeare's time had come to mean a heavy ram. So when Falstaff exclaims, "fillip (strike) me with a three-man beetle" (Henry IV Part 2, I.ii.230), he means a sledgehammer that would take three men to lift. And when Petruchio curses his servant for being a "whoreson, beetle-headed, flap-eared knave" (The Taming of the Shrew, IV.i.143), he means no more than "block-headed".
However, the figurative sense may derive from the insect. When Mercutio, referring to his face-mask, says, "Here are the beetle brows shall blush for me" (Romeo and Juliet, I.iv.32) or Horatio talks of the cliff "that beetles o'er his base into the sea" (Hamlet, I.iv.71), both usages refer to something that overhangs or is prominent, probably an allusion to the tufted antennae found in some species of beetle.
David Crystal is author, with Ben Crystal, of Shakespeare's Words (Penguins)