i Register
In some senses, overburthen is marked as archaic. Watch for register when choosing this word.
noun
Archaic form of overburden.
The over burthen of these expenses falls upon the sale of the commodity, and lessens the consumption.
1832, John W. Colenso, "A Description of Happy-Union Tin Stream-work at Pentuan" in Transactions of the Royal Geological Society of Cornwall, T. Vigurs, Vol. 4, p. 38 https://books.google.ca/books?id=TjK_HG0RIdoC&printsec=frontcover#v=onepage&q&f=false The overburthen contains no sea-sand, but is composed of silt and river gravel […] .
verb
Archaic form of overburden.
In short the last man in the world, one would think, to be overburthened with thoughts, especially those perilous thoughts that must needs belong to a conspirator in any serious project, or even to the underling of such a conspirator.