flea in one's ear, a - 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.
An annoying hint or a stinging rebuke, as in He has a flea in his ear about their relationship, or If he doesn't bring the right equipment, I'll put a flea in his ear. This expression originated in French and has been used in English since the 1400s.