(must) v.
A defective verb now used only as an auxiliary followed by the infinitive without to, or elliptically with the infinitive understood, to express:
1. Obligation or compulsion: Must you go? I must.
2. Requirement: You must be healthy to be accepted.
3. Probability or supposition: You must be tired.
4. Conviction or certainty: War must follow. ♦ A past conditional is formed by placing the following verb in the perfect infinitive: He must have gone.
—noun
1. Anything that is required or vital.
2. A news item or other material that must be printed: usually marked must.
—adjective Important and essential: a must book. [OE moste, pt. of mōtan may]

(must) noun
Mustiness; mold.
—v.t. & v.i. Obs. To make or become musty. [Back formation <MUSTY¹]

(must) noun
1. The expressed unfermented juice of the grape.
2. Unfermented potato pulp. [OE <L mustum (vinum) new wine]

(must) noun
1. A state of dangerous frenzy, related to sexual excitement, associated with adult male animals, especially elephants.
2. An elephant in this condition.
—adjective Being in a state of must. Also spelled musth. [<Hind. mast drunk, lustful]

(must) noun Scot.
Musk, especially in the form of hair powder.