Mojo has been a word used by black musicians to describe the spell thant makes their music work, and has passed into general use for any charm or spell. In musical terms, it means what ever you want it to mean - the music itself, the sexual prowess of the musician, anything. best use of the word is Jim Morrison's line in the song 'L.A. Woman' when he chants "Mr Mojo risin' ...." which apart from being a great line in a superb song, happens to be an anagram of his name.
i was always led to believe that originally it was a magic charm associated with voodoo - it would be a small bag worn round the neck containing a collection of small magic objects.
and it certainly wasn't invented by austin powers!
Agree with Andy, a lot of guitars being sold on ebay boast about how much mojo they've got (I'm not kidding). I don't agree with him about LA Woman being a great song though...
The word itself is probably West African in origin and originally meant magic or the ability to cast spells, particularly in a figurative sense - as Andy says above - used of their music by black jazz musicians in the 1920s.
Being the age I am , I have a lot of singles from the early seventies on the Mojo label as well... including Kool & the Gang when they were a good funk/jazz band.
Andy Hughes is almost correct, except that he has the transfer backwards. the word in its magical contect is very old and was adopted by Musicians not vice versa.