When a person (typically a girl), receives so many compliments that the value of successive compliments decrease.
John: Ava, you’re so pretty!
Ava: Okay
John: Do you not like it when I say that?
Ava: No, it’s just that I get told I’m pretty all the time so the compliment doesn’t feel as special.
John: Oh I see, being called pretty has lost it’s effect due to compliment inflation.