drop in one's tracks - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Collapse from weariness or illness; also, die suddenly. For example, I packed all day until I could have dropped in my tracks, or Grandfather's died; he just dropped in his tracks. The phrase in one's tracks has meant “where one is at the moment,” and by extension, “instantly,” since the early 1800s.