VERBS, AUXILIARY. Must = "is to;" original use ofMust (E. E. moste) is the past tense of the E. E. present tense mot, which means "he is able," "he is obliged." From meaning "he had power to do it," or "might have done it," the word came to mean "ought," and it is by us generally used with a notion of compulsion. But it is sometimes used by Shakespeare to mean no more than definite futurity, like our "is to" in "He is to be here to-morrow."
So, or nearly so, probably in
“He must fight singly to-morrow with Hector, and is so prophetically
proud of an heroical cudgelling that he raves in saying
And somewhat similar, without the notion of compulsion, is the use in M. of V. iv. 1. 182; M. N. D. ii. 1. 72. It seems to mean "is, or was, destined" in
“Descend, for you must be my sword-bearer.
So “A life which must not yield
“And I must be from thence.
To one of woman born.” Ib. v. 8. 12.