One of the mysteries of the English language finally explained.
Without reference to a book or script; from memory; (in early use) by improvisation, extempore.
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.
Early 18th century. From off + book.
off book/ˈɒf bʊk/
In this article we explore how to impress employers with a spot-on CV.