must

1 must /ˈmʌst/ verb
1 must
/ˈmʌst/
verb
Britannica Dictionary definition of MUST
[modal verb]
1 
a  somewhat formal in US English — used to say that something is required or necessary
◊ Note the difference in meaning between must not and not have to.
b  — used to say that something is required by a rule or law
2 
somewhat formal in US English — used to say that someone should do something
3 
— used to say that something is very likely
4 
— used in various phrases to emphasize a statement
5 
somewhat formal in US English — used in questions that express annoyance or anger

if you must

— used to say that you will allow someone to do something even though you do not approve of it

if you must know

— see 1know
2 must /ˈmʌst/ noun
plural musts
2 must
/ˈmʌst/
noun
plural musts
Britannica Dictionary definition of MUST
[count]
: something that is or seems to be required or necessary — usually singular