Definition of off book in US English:

off book

adverb

  • Without reference to a book or script; from memory; (in early use) by improvisation, extempore.

adjective

  • 1Finance. Not properly declared for tax or accounting purposes.

  • 2Theatre and Film. In predicative use: (of an actor) performing without a script, having memorized his or her lines.

Origin

Early 18th century. From off + book.

Pronunciation

off book

/ˈɒf bʊk/