must

The use of must informally as a noun meaning 'something that must be done or had, or that should not be missed', dates from the 1890s in American use. In the earlier part of the 20th century, it was often written in inverted commas as being not quite pukka in serious or supposedly serious contexts, and this practice still sometimes occurs:

A film and a song made the Trevi Fountain a 'must' for tourists—Guardian, 1973.

Now, the idiom has moved a stage further in being used attributively (before a noun: This is a 'must' book), and in phrasal adjectives such as must-have, must-see, etc. (in which must is a verb and not a noun):

The King George is, self-evidently, a royal occasion, one of the 'must-see' races—Observer, 2007).

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