Sub-rosa is an adjective that describes an event as having been accomplished in secret or hiding. It is one of the few sub- words that has a hyphen as the preferred spelling inside North America. In British English the preferred spelling is two words, without the hyphen.
Sub rosa, without the hyphen, can also be an adverb that describes an action as having been in secrecy. This non-hyphenated form is the preferred form everywhere.
This follows the general hyphen rule that compound adjectives are hyphenated for clarity in what they modify.
The term dates back as far as the 1650s. It is Latin and literally means under the rose, a symbol for secrecy. It has been in the English language long enough that italicizing is not necessary.
Examples
Israel’s military has been waging a sub-rosa tug-of-war with the prime minister for years over strategic direction on several fronts, including relations with the Palestinians. [Jewish Daily Forward]
By not marching we’d covertly parlay our reputation for street violence and idiotic racial aggro into a position of genuine if sub rosa influence over the policies of mainstream politicians, thus continuing, so to speak, our ideological march through the institutions. [London Evening Standard]
It would be a healthy and courageous step if the Arabs institutionalized what they are already doing sub rosa with Tel Aviv. [Huffington Post]
Every day, Wirathu’s Facebook page is updated with the details of alleged new Muslim atrocities – murders, bomb attacks and, above all, rape, which he claims is used by Muslims not only as an instrument of terror, but as a way of carrying out a sub rosa colonisation of the country. [British GQ]