Source : Webster's Revised Unabridged Dictionary (1913)

Burthen \Bur"then\, n. & v. t.
   See {Burden}. [Archaic]

Source : WordNet®

burthen
     n : a variant of `burden'
     v : weight down with a load [syn: {burden}, {weight}, {weight
         down}] [ant: {unburden}]