Arduous

1. a. Attended with great labor, like the ascending of acclivities; difficult; laborious; as, an arduous employment, task, or enterprise.
2. a. Steep and lofty, in a literal sense; hard to climb.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.