Definitions

1. verb

an archaic second person singular past tense of ‘must’ meaning ‘had to’ or ‘was obligated to’

“He mustnt have eaten breakfast that morning.”

2. verb

an archaic second person singular past tense of ‘must’ used in negative statements meaning ‘did not have to’ or ‘was not obligated to’

“You mustnt have tried, it was impossible.”

3. verb

a negative imperative of ‘must’ used in the second person singular, meaning ‘do not have to’ or ‘are not obligated to’

“You mustnt worry about it.”