A really hip, slang way to say what's up man? Applicable in any situation; to ask what are you doing? how are you doing? what would you like to do? what are you planning on doing? or just a general 'how's life?'
American in origin but became popular in the UK.
Derived from adjective "mousey," a similar word with multiple meanings including icky, lame, suspect and/or grayish brown.