lose train of thought

lose (one's) train of thought

To be unable to resume one's previous progression of thought. A: "Therefore, it is critical to remember that—" B: "Professor! Will this be on the final exam?" A: "Yes, Stephen. Shoot, now I've lost my train of thought." And so you see, by collecting such data, we can make an inference to the, um… the… What am I trying to say? I lost my train of thought.
See also: lose, of, thought, train
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

lose one's train of thought

Fig. to forget what one was talking or thinking about. Excuse me, I lost my train of thought. What was I talking about? Your question made the speaker lose her train of thought.
See also: lose, of, thought, train
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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