Dictionary.com
definitions

a must

A necessity; a requirement. For example, The Louvre is a must for visitors to Paris, or This book is a must for serious students of English. [Late 1800s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.

Related Words for a must

obligatory, restraint, pressure, motive, necessity, driving, spring, violence, goad, repression, coercion, suppression, hang-up, compulsion, spur, duress, monkey, impulsion, conventional, correct