secretly

privately, covertly, obscurely, darkly, surreptitiously, furtively, stealthily, clandestinely, underhandedly, slyly, behind one's back, intimately, personally, confidentially, between you and me, in strict confidence, in secret, behind the scenes, on the sly, behind closed doors, quietly, hush-hush*; see also secretive.

Antonyms openly*, obviously, publicly.