clandestinely
clan-des-tine-ly
adverb
In a secret or surreptitious manner, often to avoid detection or disapproval.
"They met clandestinely to discuss the secret plan."
Thesaurus
Synonyms: surreptitious, hugger-mugger, secret, clandestine, hush-hush, cloak-and-dagger, undercover, covert, hole-and-corner, underground