That which is requisite, necessary, or indispensable. I know this book is a difficult read, but it's an absolute must if you enjoy literature. Taking the introductory course is a must if you want to sign up for the advanced class.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.