Hi,
What does this sentence mean?
He has a flea in his ear about their relationship.
Does it mean "He hates their relationship"?
a disconcerting rebuke or rebuff: The next time he shows his face around here he'll get a flea in his ear.
an alerting hint. I will put a flea in his ear about what is happening in this place
He has a flea in his ear about their relationship.
I understand "a flea in his ear" in this example to mean that he is annoyed or bothered by the relationship, and that he has probably been complaining about it.
If the "alerting hint" that Nersi mentions is appropriate here then it might suggest that he has been warned about some aspect of the relationship.
The phrase can also have other slightly different meanings.
"She didn't like his attitude and sent him away with a flea in his ear" means that she criticized him, told him something he didn't like hearing, told him off.
not a teacher
No, it's universal in the English-speaking world.
It's always used metaphorically in the ways shown above. The way you used it in post #1 is just about the only way in which it is not used.
You don't have a flea in the ear - you get one, come away with one, give somebody one, or send somebody away with one.
Rover