The yeah comes from the Soldier, one of the playable classes in Valve's Team Fortress 2, first released in 2007. In the game, the Soldier is a loud, overconfident American mercenary, and his short affirmations are often used by players to celebrate kills or agree with teammates. Bass-boosted versions later spread across soundboards and short-form video, where the distorted low end turned a simple approval into a louder, funnier reaction. It remains popular because the word is instantly understood, even without any game context, and its punchy length fits easily into memes and edits.