In a surreptitious manner; stealthily, furtively, secretly.(adverb)
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Use "surreptitiously" in a sentence
"If you see a group show up, take their picture, surreptitiously is safer because some of these thugs will hurt you."
"Now she has realised that by continuing dishonesty Blair and then Flash have succeeded in surreptitiously nudging the state forward but have forfeited the debate, which they have lost."
"His clothes were rough and half soaked by the rain that had been falling, while it became apparent as we talked that he had landed surreptitiously from a Dutch fishing-boat early that morning and had not dared to show himself."