I used to hear this term used often when I was a technician with a computer company. We would use the term when we didn't know what to do to repair something or why we chose to replace a part on a hunch.
Technician Bob: I didn't know. I just took my best SWAG.
Darrel: You got so much dick in your ass
Assface Magee: Fuck you, SWAG
Me: 'That's because you're a fucking retard trying to be like the rest of the try hards thinking their dumb ass have swag.'
The word quickly made its way to the states and has ever since become the catchphrase of douchbags and tools everywhere.
Person 2: "You're a dick"