World Wide Words has an excellent discussion of the origin of this term and how the meaning has changed completely over the centuries. It began as “meet” in medieval England, and it was often spelled “mot” or “mote.” When something was put up for discussion and debate to reach a decision it was “mooted.” Law students later began using the word “moot” to describe their debates of hypothetical issues. The outcome was unimportant since the issues were invented to help them improve their debating skills. The result was “…a curious shift in which the sense of ‘open to debate’ has become ‘not worth debating’.” The discussion also warns against using the incorrect “mute” to substitute for “moot.”