Mack truck

noun3

1. •
   A motor vehicle produced by Mack Trucks, especially a semi-trailer.

   • their hearts were too small to pump blood to their huge bodies. It was like trying to run a Mack truck on a Volkswagen engine, and their hearts gave out. — 2005
2. •
   A semi-trailer of any brand.
3. •
   (figurative) Any unstoppable or enormous thing coming through.

   • I sprung out of bed and was shoved back by a Mack truck of a headache. I powered through and felt my way to the door, focusing on a sliver of grayish light glowing through the curtains. — 2011