must1
Pronunciation: /mʌst/
modal verb ( past had to or in reported speech must)
- 1be obliged to; should (expressing necessity): you must show your ID card the essay mustn’t be over 2,000 words she said she must be going
- expressing insistence: you must try some of this fish if you must smoke you could at least go in the living room
- used in ironic questions expressing irritation: Charlotte, must you put spanners in the works?

Origin:
Old English mōste, past tense of mōt 'may', of Germanic origin; related to Dutch moeten and German müssen

Grammar
A modal auxiliary verb used to express the speaker's view of how necessary or desirable an event may be: ‘I must go,’ breathed Stefania, and was gone. See also modal meanings.