1. verb /mʌst,məs(t)/
a) to do with certainty;
If it rained all day, it must be very wet outside.
b) to do as a requirement;
You picked one of two, and it wasnt the first: it must have been the second.
2. noun /mʌst,məs(t)/
a) Something that is mandatory or required
If youll be out all day, sunscreen is a must.
b) Something that exhibits the property of being stale or musty
Wikipedia foundation.