a must

a must idiom
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.

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