Plants vs. Zombies is a tower defense game developed by PopCap Games and first released in 2009, where players use plants to stop waves of zombies crossing a lawn. The zombies are defined as much by their shambling movement and groans as by their appearance, and those vocal effects became part of the game's identity. As gameplay clips, fan animations and meme edits spread online, this groan was lifted as a standalone reaction sound for any slow, brainless entrance, and it remains closely tied to the original lawn defense battles.