In secrecy; not openly:
— Met secretly to discuss the invasion plans.
— The children secretly went to the movies when they were supposed to be at the library.

synoniemen: in secret, on the Q.T., on the QT.

Roget 526: by a side wind; sub silentio [Lat.]; in the background; behind the scenes, behind one's back; on the tip of one's tongue; secretly etc. 528; between the lines.   

Roget 528: secretly etc. adj.; in secret, in private, in one's sleeve, in holes and corners; in the dark etc. adj.. januis clausis [Lat.], with closed doors, a huis clos [Fr.]; hugger mugger, a la derobee [Fr.]; under the cloak of, under the rose, under the table; sub rosa [Lat.], en tapinois [Fr.], in the background, aside, on the sly, with bated breath, sotto voce [Lat.], in a whisper, without beat of drum, a la sourdine^†.    behind the veil; beyond mortal ken, beyond the grave, beyond the veil; hid from mortal vision; into the eternal secret, into the realms supersensible^†, into the supreme mystery.    in confidence, in strict confidence, in strictest confidence; confidentially etc. adj.; between ourselves, between you and me; between you and me and the bedpost; entre nous [Fr.], inter nos, under the seal of secrecy; a couvert [Fr.].    underhand, by stealth, like a thief in the night; stealthily etc. adj.; behind the scenes, behind the curtain, behind one's back, behind a screen etc. 530; incognito; in camera.    ... meer laten zien

Not openly; inwardly.