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secretly  
behind closed doors, behind (someone's) back, clandestinely, confidentially, covertly, furtively, in camera, in confidence, in one's heart, in one's heart of hearts, in one's innermost thoughts, in secret, on the fly  (slang, chiefly Brit.) on the q.t.  (informal) on the sly, privately, quietly, stealthily, surreptitiously, under the counter, unobserved