"To forgive is the most arduous pitch human nature can arrive at." When Richard Steele published that line in The Guardian in 1713, he was using arduous in what was apparently a fairly new way for English writers in his day: to imply that something was steep or lofty as well as difficult or strenuous. Steele's use is one of the earliest documented in English for that meaning, but he didn't commit it to paper until almost 150 years after the first uses of the word in its "strenuous" sense. Although the "steep" sense is newer, it is still true to the word's origins; arduous derives from the Latin arduus, which means "high," "steep," or "difficult."

Every summer, right before the beginning of the new school year, the football team begins its season with "Hell Week," an arduous six days of conditioning and training.

 

"The mission has been long and the road arduous for Israeli nonprofit SpaceIL, which has, in some iteration or another, been working on the concept of a lunar lander for nearly a decade."

— Chabeli Herrera, The Orlando (Florida) Sentinel, 20 Feb. 2019

What 4-letter noun beginning with "sl" can refer to a prolonged arduous task or to a hard, dogged march?