BESOM is a 5 letter word that starts with B.

1. \Be"som\, n. [OE. besme, besum, AS. besma; akin to D.
   bezem, OHG pesamo, G. besen; of uncertain origin.]
   A brush of twigs for sweeping; a broom; anything which sweeps
   away or destroys. [Archaic or Fig.]
   
         I will sweep it with the besom of destruction. --Isa.
                                                  xiv. 23.
   
         The housemaid with her besom.            --W. Irving.
   
   

2. \Be"som\, v. t. [imp. & p. p. {Besomed}.]
   To sweep, as with a besom. [Archaic or Poetic] --Cowper.
   
         Rolls back all Greece, and besoms wide the plain.
                                                  --Barlow.
   
   

the rendering of a Hebrew word meaning sweeper, occurs only in Isa. 14:23, of the sweeping away, the utter ruin, of Babylon.