My magic charm is working.

In the early 20th century mojo meant voodoo or magical power; more recently this has been extended to mean power of influence of any kind. The term was widely used in the US black communities then and in 1926, Newbell Niles Puckett published this definition in his Folk Beliefs of the Southern Negro:

"The term mojo is often used by the Mississippi Negroes to mean 'charms, amulets, or tricks', as 'to work mojo' on a person or 'to carry a mojo'."

McKinley Morganfield, a.k.a. Muddy Waters, would have heard work mojo as he was growing up in Mississippi and used it in the title of his well-known blues classic Got my mojo working in 1956. This is probably the first time that the phrase was committed to record - either vinyl or paper.

Got my mojo working, but it just won't work on you
Got my mojo working, but it just won't work on you
I wanna love you so bad till I don't know what to do

I'm going down to Louisiana to get me a mojo hand
I'm going down to Louisiana to get me a mojo hand
I'm gonna have all you women right here at my command

Mojo is also recorded as meaning cocaine/heroin etc. In Pollock's The Underground Speaks, 1935 he records Mojo as "any kind of poisonous habit-forming narcotics (dope)".