sub rosa
[adverb]
Synonyms: secretly, by stealth, clandestinely, covertly, furtively, hugger-mugger, in camera, privately, stealthily, surreptitiously
[adjective]
Synonyms: secret, clandestine, covert, hugger-mugger, hush-hush, stealthy, surreptitious, undercover, underneath, under-the-table

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Antonyms: aboveboard

[TahlilGaran] English Synonym Dictionary ▲

sub rosa adjectival & adverbial phr. [sʌb 'rǝʊzǝ] M17.
[Latin, lit. 'under the rose' (as an emblem of secrecy).]
Of communication, consultation, etc.: (given, told, etc.) in secrecy or confidence.

[TahlilGaran] English Dictionary ▲