a single rope; a piece of rope
Literal meaning: one strand of rope. Rarely used outside the idiom 一筋縄ではいかない.
He struggled so wildly that a single rope couldn't hold him.
ordinary means; straightforward method
Almost exclusively used in the negative idiom 一筋縄ではいかない, meaning 'not straightforward', 'tricky', or 'hard to deal with'. The word itself is not used independently.
See also: 一筋縄ではいかない
He is a tough person to deal with.
This problem is not easily solved.