落 (luò): To fall, to drop.
井 (jǐng): A well (for water). The character is a pictograph of a wellhead seen from above.
下 (xià): Down, downwards, to put down.
石 (shí): Stone, rock.
When combined, the characters create a literal and unforgettable story: 落 (to fall into a) 井 (well), [and someone] 下 (drops) 石 (stones). This sequence paints a clear picture of someone taking advantage of another's complete helplessness to inflict further harm.
`落井下石` is a strongly negative and accusatory term. It is widely understood and used in various contexts, from casual conversation to formal media reports, to condemn unethical actions.
In Business and Politics: It's frequently used to describe competitors who exploit a rival's crisis, or politicians who launch attacks on an opponent embroiled in a scandal. It implies unfair and predatory tactics.
In Personal Relationships: It describes a “friend” who, upon learning of your troubles (e.g., a breakup, job loss), spreads rumors or takes advantage of the situation. It is one of the worst accusations you can level at a friend, signifying a deep betrayal.
Formality: While it's a classical idiom (`chengyu`), it's used in both formal writing (news, essays) and everyday spoken Mandarin when a situation warrants such strong condemnation. It is not a light or casual expression.