偷 (tōu): To steal; to do something secretly or sneakily.
梁 (liáng): A horizontal support beam, especially a main roof beam. This is a critical structural element.
换 (huàn): To change, to exchange, to substitute.
柱 (zhù): A vertical pillar or column. Also a critical support element.
The characters combine to create the vivid picture of “stealing a beam and replacing it with a pillar.” In a more general sense, it means replacing one crucial thing (the beam) with another (the pillar, or often, an inferior beam). The act is done with stealth (偷) and involves a deceptive substitution (换) of a core component, thereby compromising the entire structure.
偷梁换柱 is a very active and relevant idiom in modern China.
Business and Commerce: This is its most common usage today. It's frequently used in news reports and daily conversation to describe corporate or commercial fraud.
Using substandard steel in a construction project.
Selling food products with cheaper, sometimes harmful, ingredients.
Selling a device with a counterfeit battery or processor inside.
Politics and Bureaucracy: It can describe a political maneuver where the spirit of a law or policy is subverted by changing key personnel or altering crucial definitions behind the scenes.
Everyday Life: In a less serious tone, it can be used to describe minor acts of deception.
Accusing a sibling of swapping your new video game with their old, scratched one.
Joking that a restaurant 偷梁换柱-ed the beef in your noodle soup with a cheaper meat.
The connotation is almost always negative, implying dishonesty and a calculated intent to deceive.