The idiom "fish rot from the head down" means that a person or thing is infected or corrupt from the top down, usually due to poor leadership or an unhealthy influence. It is often used to describe a situation where a problem starts at the top and spreads throughout the organization.

The idiom "from pillar to post" means that someone is constantly moving around or changing places rapidly, without any clear purpose or direction. It can also indicate a lack of stability or continuity in one's life.

The idiom "ducks and drakes" refers to a set of objects or situations that are mutually exclusive or have opposite characteristics. It is often used to describe two things that are opposite in some way, such as happy and sad or hot and cold.

• The idiom "X marks the spot" is used to indicate the location of something or someone that is important, valuable, or of interest. It is often used to describe a hidden treasure or a secret that has been revealed.

"By chance" is an idiom that means something happens unexpectedly or accidentally, without any intention or planning. It's used to describe a situation where the outcome was not predetermined or intended.