TIL that “Re:” which is often used on subject line of emails is Latin for “on the matter of”. I used to think it’s an abbreviation of ‘regarding’ or ‘reply’.

https://blog.duolingo.com/latin-alive-and-ready-to-be-learned/

https://www.reddit.com/r/todayilearned/comments/154x4y0/til_that_re_which_is_often_used_on_subject_line/