listen in

/L IH1 S AH0 N IH0 N/
verb
  1. 1

    To listen without participating.

    โ€œDo you mind if I listen in on your meeting tomorrow?โ€

  2. 2

    To eavesdrop; to listen secretly.

    โ€œYou should close the door for conversations like that. You never know who might be listening in.โ€

Source: https://en.wiktionary.org/wiki/listen%20in