Use 'buff up' when referring to improving knowledge in a specialized subject or skill.

Subject-Specific

'Buff up' implies preparation for a specific event or requirement, such as an exam or interview.

Preparation Essential

Common in casual or informal conversations, often used when giving advice.

Often used to describe the act of physically polishing something until it looks new or shiny.

Implies taking time and effort to care for an object, suggesting pride in ownership.

Suggests an instant visual improvement after the action is taken.

Mainly related to improving physical appearance, especially getting more muscular.

Highlights the hard work and dedication required to physically transform one's body.

Implies not only aesthetic improvement but also increasing strength and stamina.