Where I got my nickname should be fairly obvious. I was first called "Mojo" in the 9th grade by a fellow named Frank Stubblefield, who was talking about HoJo's (Howard Johnson's restaurant nickname) at the time. He looked at me and said, "Mojo!" It stuck ever since.
It might not have stuck so well if I hadn't adopted "Mojo" instead of "Morris" for my name on ham radio.
Being Mojo, I tend to hear about other mojos in the world. It turns out to be a very rich word with a long history.
There is one meaning for mojo that appears to be the original and oldest I can find: A mojo is a voodoo magic charm. It can be specifically a fertility charm, or just a generic word for voodoo magic. If you have a collection of charms, you might also keep them in a "mojo bag," which I'm told is made from cat parts. In fact the word "mojo" is pidgin for the english word "magic."
So when an old blues singer sings of having his mojo working, he means he's likely to be lucky with the ladies tonight. If his baby's got a mojo, it means he's going to remain faithful to her -- can't resist the magic! I believe the first reference to mojo in song came from Muddy Waters, who recorded "The Mojo Blues."
A later reference to mojo comes from The Beatles in "Come Together" from Abbey Road ("He got ... mojo filter").
But by far the most notable reference is from Jim Morrison and The Doors in LA Woman, "Mr. Mojo Risin' ... risin' risin'." This is the one most people think of when they hear the word mojo. Mojo will always be closely associated with Morrison -- the expression "mr. mojo risin" is an anagram for Jim Morrison. Morrison said that he would be reincarnated as "Mr. Mojo Risin'."
However mojo is still a popular word in other contexts:
Morris "Mojo" Jones