open book

noun

1. someone or something easily understood or interpreted; something very clear.

   The child's face is an open book.

open book

noun

1. a person or thing without secrecy or concealment that can be easily known or interpreted

Word History and Origins

Origin of open book¹

First recorded in 1850–55

Idioms and Phrases

Something or someone that can be readily examined or understood, as in His entire life is an open book. This metaphoric expression is often expanded to read someone like an open book, meaning “to discern someone's thoughts or feelings”; variations of this metaphor were used by Shakespeare: “Read o'er the volume of young Paris' face,” (Romeo and Juliet, 1:3) and “O, like a book of sport thou'lt read me o'er” (Troilus and Cressida, 4:5). [Mid-1800s] For an antonym, see closed book.