onerous

Definitions:

Onerousness.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
Burdensome; oppressive.
- Newage Dictionary DB (adjective) (adj.) (adjective)
Heavy; burdensome; oppressive; in Scotch law, being of mutual advantage.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
Burdensome; weighty; as, an onerous duty.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
Onerously.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adverb) (adv.) (adverb)

Usage examples:

We know of no class who have more onerous duties to discharge than magistrates of districts and their subordinates.
- Life and Work in Benares and Kumaon, 1839-1877 by James Kennedy
And Katy, while still serving as main spring to the whole, had plenty of time to write her notes, open her wedding presents, and enjoy her friends in a leisurely, unfatigued fashion which was a standing wonderment to Cecy, whose own wedding had been of the onerous sort, and had worn her to skin and bone.
- Clover by Susan Coolidge
The Kogisho was composed mostly of the retainers of the Daimios, for the latter, having no experience of the earnest business of life, " were not eager to devote themselves to the labors of an onerous and voluntary office."
- The Constitutional Development of Japan 1863-1881 by Toyokichi Iyenaga