1
general::
Also, rub it in. Harp on something, especially an unpleasant matter, as in She always rubs in the fact that she graduated with honors and I didn't, or I know I forgot your birthday, but don't keep rubbing it in. This idiom alludes to the expression rub salt into a wound, an action that makes the wound more painful; it dates from medieval times and remains current. [Mid-1800s] Also see RUB SOMEONE'S NOSE IN IT.
American Heritage Idioms
2
general::
Phrase(s): rub something in
Fig. to keep reminding one of one’s failures; to nag someone about something. • I like to rub it in. You deserve it! • Why do you have to rub in everything I do wrong?
McGrawhill's American Idioms And Phrasal Verbs