What is the meaning of Burthen?

Archaic form of burden.

1. The tonnage of a ship based on the number of tuns of wine that it could carry in its holds.
2. Rarely used musical tempo, meaning slow and ponderous (e.g., moving under a burden); similar to largo.
3. A section of music with the tempo mark burthen.

The tonnage of a ship based on the number of tuns of wine that it could carry in its holds.

Rarely used musical tempo, meaning slow and ponderous (e.g., moving under a burden); similar to largo.

A section of music with the tempo mark burthen.

Archaic form of burden.

Source: wiktionary.org