Literally: barbarian/flatten* – steep/dangerous – one – joint/section/occasion
Alternately: You should stand firm to your principles regardless of whether things are calm and easy, or dangerous and hard. Don’t sell out your values, no matter the circumstances.
Notes: This particular rare compound comes to us from Chinese antiquity, attributed to a Song-dynasty scholar called Ouyang Xiu (欧陽脩, Ouyou Shuu in Japanese).
*The character 夷 can refer to the indigenous people of northern Japan, such as the Emishi and Ainu. It can also serve as a derogatory term for foreigners or “unsophisticated” people, looked down on by the self-congratulatory, aristocratic, capital-dwelling speaker. For some utterly mysterious reason, the same character can also mean “to subjugate,” “to put down a rebellion,” “to level [something] out.”
This yojijukugo uses the meaning of “level,” or by extension “easy, peaceful.” 夷険 thus means something like “good times and bad,” while 一節 refers to having a single set of principles, in this compound of compounds.