It's a short hand way and abreviation to say "Also Known As"

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed

Abbreviation of 'also known as'. Utilized when using an alternate name for someone/thing.

Is a thing most Dj´s and rapper use these days it means ( also known as )

How to introduce a euphemism. Saying something while meaning something else.
Used in a sarcastic manner