We investigate experimentally whether American English adult speakers are influenced in their interpretation of mustn’t by pragmatic context (contexts favoring lack of necessity/necessity not to readings) and/or the semantic properties of the verbal complements of the modal (verbs denoting events in the physical realm vs. verbs expressing undesirable mental activities). In an experiment combining a forced choice task and a gradient acceptability task, participants saw sentences containing mustn’t and physical events/negative mental activities in lack of necessity/necessity not to contexts (e.g., You mustn’t worry. The woman will give you money) They had to choose the most suitable interpretation of mustn’t ('it is necessary not to'/'it is not necessary' interpretations). They then had to rate the acceptability of the sentences containing mustn’t in context on a Likert scale from 1 to 7. We find that participants split into two groups: an Interdiction Group, which always treated mustn’t as expressing interdiction, and a Variation Group, which tended to interpret mustn’t as lack of necessity when the context favored such a reading and when the verbal complement the modal combined with was a negative mental activity. We argue that the lack of necessity reading of mustn’t is obtained via pragmatic weakening from its primary interdiction reading, and that this process is sensitive to context, as well as to the cognitive difficulty of imposing or forbidding mental (but not physical) activities to others.