koso-koso
Speaking or acting in a secretive, furtive way. The sound and manner of people whispering behind someone's back.
💡Coworkers gossiping in the break room — 'They were talking koso-koso when I walked in.'
二人がこそこそと何か話している。
The two of them are talking koso-koso about something secretly.
Why are you two always talking ___ behind my back?
なんで二人はいつも○○と内緒話をしてるの？