How did Vatican City get its nickname?

Answer 1

It is also known as the Holy See.

Clarification:

Actually, the Holy See refers to the Pope and other members of the Vatican hierarchy. The Holy See and the Vatican are two separate entities. Vatican City is sometimes called simply 'The Vatican.'

Answer 2

I have never heard of a nickname for Vatican City. It gets its name from the hill upon which it was built - the Mons Vaticanus.