The Collaborative International Dictionary of English v.0.48:
Burthen \Bur"then\, n. & v. t. See Burden. [Archaic] [1913 Webster]WordNet (r) 3.0 (2006):
burthen n 1: a variant of `burden' v 1: weight down with a load [syn: burden, burthen, weight, weight down] [ant: disburden, unburden]