This sound is associated with Garry's Mod, the sandbox game built on Valve's Source engine and released by Facepunch in 2006. Because the game is famous for physics props, ragdolls, and machinima-style chaos, the flopping body impact audio became instantly recognizable to players. It spread through gameplay videos, YouTube memes, and short edits where characters crumple or fly into walls. People still use it because it communicates an over-the-top physics collapse in a split second, even when the original footage is from a different game.