1530s, "hard to accomplish, difficult to do, attended with much labor," from Latin arduus "high, steep," also figuratively, "difficult, hard to reach," from PIE root *eredh- "high" (see ortho-). Literal sense of "high, steep, difficult to climb" is attested in English from 1709. Related: Arduously; arduousness.
What is arduous requires more energy and endurance, and is less within the reach of common powers, than what is hard. Its primitive meaning of steep climbing is still felt in it, and makes it suggestive of severe and protracted effort. [Century Dictionary]