しのびやかに (translations)→ insidiously stealthily
そっと (translations)→ calmly carefully cautiously covertly delicately discreetly ease furtive furtively gently gingerly guardedly lightly little loosely politely quick quickly quiet quietly silently slight slowly softly stealthy tenderly tentative
ひそかに (translations)→ mentally private privately surreptitiously unadvertised
人知れず (translations)→ secret undiscovered
内緒で (translations)→ semi-privately
内心 (translations)→ actually conscience enjoy enormously inside internal inward inwardly sneaking truth within