Being Corrected Incorrectly

It drives me nuts to be corrected by someone, especially a “superior,” when I know they are incorrect. I am too polite to tell people they are wrong, because it feels rude, but afterwards I will agonize over the incident for months. This happened twice recently, once when someone corrected me that “quarterly” means every four months not three (which is incorrect), and once when my boss used “expediate” (not a word) instead of “expedite.” I didn’t say anything but now I am gritting my teeth over it. How does one point that out (especially to one’s boss) without being rude? It drives me crazy.