Definition of on book in English:

on book

adverb

Theatre
  • With reference to a script; not from memory.

adjective

Theatre
  • In predicative use: (of an actor) reading from a script during rehearsal or performance, not having memorized his or her lines.

Origin

1980s. From on + book, after off book.

Pronunciation

on book

/ˈɒn bʊk/