Latin, ablative of res 'thing'.
It is often said that, strictly speaking, re should be used in headings and references, as in Re: Harrison versus Ortiz, but not as a normal word meaning ‘regarding,’ as in thanks for your letter re the job postings. However, the evidence suggests that re is now widely used in the second context in official and semiofficial contexts, and is now generally accepted. Be aware, however, that in certain formal contexts, if re is used in mid-sentence, some readers may regard it as business jargon or an inappropriate legalism. Often, concerning or about would be just as clear (and less likely to annoy).
Middle English re, representing (as an arbitrary name for the note) the first syllable of resonare, taken from a Latin hymn (see solmization).