surreptitiously surreptitiously /ˌsɝːəpˈtɪʃəsli/

adverb

1. () In a surreptitious manner  ( sneakily , clandestinely , surreptitiously )
   in a surreptitious manner
    He was watching her surreptitiously as she waited in the hotel lobby
   he was watching her surreptitiously as she waited in the hotel lobby