It is a modern phrase born from social media, specifically twitter. The handle is the text after the '@' sign in the user ID you get from twitter and use to sign your tweets.
And since the nr of Twitter users have grown explosively it is hard to get a good and simple handle like 'BossMan' or 'Angel' it has to be more complex than that 'bossman1234567890' or 'AngelOfTheMorningqwerty'
No. From what I have read "Old Charlie" refers to Darwin. When he proposed the and expanded the idea of evolution he stole the handle on human existence held by creationists.