When combined, the characters paint a vivid picture: “To turn your back on trust (背信) and abandon/discard righteousness (弃义).” The structure highlights a two-part moral failure: the betrayal of a specific trust and the abandonment of general moral principles.
The gravity of 背信弃义 (bèixìnqìyì) is rooted in the immense value placed on 信 (xìn) and 义 (yì) in Chinese culture, largely influenced by Confucianism.
Comparison to Western Concepts: A Westerner might say someone “breached a contract” or was “two-faced.”
This is a formal and very strong term. You will almost never hear it in casual, everyday conversation about minor issues.