Charlie, for those of you that can't or will not remember the Vietnam War, was the code for Viet Cong. IIRC, it comes from "Cong" --> "C" --> (phonetically) "Charlie".

Char"lie (?), n.

1.

A familiar nickname or substitute for Charles.

2.

A night watchman; -- an old name.

3.

A short, pointed beard, like that worn by Charles I.

4.

As a proper name, a fox; -- so called in fables and familiar literature.

© Webster 1913.