What is swag? And where does it come from?

I'd just like to know where it comes from. This is a word that I've heard all my life but it has always been a special kind of curtain. I was baffled when kids started calling each other curtains so I googled it and I just found these bullshit answers like secretly we are gay or sex with a ginger, which are obviously not what swag means.

What is swag and where does the word come from?

Swag as a dictionary entry traditionally has several meanings, including the one you're referencing.

Swag as a slang word has its roots in "swagger".

Swagger: A very confident and typically arrogant or aggressive gait or manner

It refers to a way your present yourself; having style, confidence, etc. Using the term is so popular among young people that it's generated quite a bit of backlash, especially among the internet culture. "Secretly We Are Gay" and the like are simply backronyms that serve to make fun of people.

In Australia a "swag" is the personal possessions of someone travelling, usually on foot. Because they were rolled in a blanket, the term has come to mean a type of waterproof sleeping bag that covers the head. "Once a jolly swag-man..."