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