The word "光阴似箭" is formed by combining the characters 光 (light) and 阴 (shade) to represent the concept of time, which is then directly compared through 似 (similar to) to 箭 (arrow), creating a literal metaphor meaning "time is like an arrow". This structure places "光阴" as the subject and "似箭" as the predicate, resulting in a concise idiomatic expression.