こそこそと (translations)→ bashfully low unobtrusively
こっそり (translations)→ creep discreetly furtively privately quietly sott-voce surreptitiously unperceived unseen
しのびやかに (translations)→ insidiously secretly
じわじわと (translations)→ inchoate ponderously relentlessly slowly steadily
そっと (translations)→ calmly carefully cautiously covertly delicately ease furtive gently gingerly guardedly lightly little loosely politely quick quickly quiet silently slight softly stealthy tenderly tentative
ひそかに (translations)→ mentally private unadvertised
