pileup I. \ˈ ̷ ̷ˌ ̷ ̷\noun (-s) Etymology:pile (VI) + up : a collision involving usually several objects and causing damage or injury < was instantly killed in an automobile pileup — Springfield (Massachusetts) Union > II. noun 1.: a jammed tangled mass or pile (as of motor vehicles or people) resulting from collision or accumulation 2.: accumulation