Undecided questions
Since when was the word 오빠 used and why was it introduced?

I have always been curious about etymology (the history of words), particularly how various words in different languages and especially how some words from one language are imported by another. Nowadays, we see many anglicisms in foreign languages, almost as if it were cool to use English-sounding words in one's own tongue. Another thing that also fascinated me was how the original languages themselves evolved to the extent that modern words are continually added to the national vocabulary.

A very small example (that most people can relate to) may be the word "오빠" which we may all have encountered from the viral Gangnam Style song. However, when I watched a movie set around the 1980s in Korea, I am pretty sure I heard a woman calling an older male friend "형" instead of "오빠", so it seems that in former times, there was no gender distinction for a term addressing an older male friend. When and why exactly was the term "오빠" introduced? Was it just meant to sound cute, especially when it is said by a younger girl? I personally don't see any real function in this word, but maybe women wanted to have a gender-suited way of addressing older male friends.