complicit
Helping a bad or wrong thing happen, even by silence.
Complicit means being involved in something wrong or questionable, even if you're not the main person doing it. When you're complicit in something, you're helping it happen through your actions, your silence, or your cooperation.
If your friend copies answers during a test and you don't say anything, you're complicit in the cheating. You didn't cheat yourself, but your silence allowed it to continue. If a student sees someone being bullied and does nothing to help or report it, they become complicit in that bullying. Being complicit doesn't require doing the bad thing yourself: it means going along with it, covering for it, or standing by while it happens.
The word carries weight because it suggests you had a choice. You could have spoken up, walked away, or refused to participate, but instead you stayed involved. An accomplice is someone who is complicit in a crime, knowingly helping the person who commits it.
People often use “complicit” when talking about larger situations too. Historians might describe people who stayed silent during injustices as complicit in those wrongs, even if they didn't actively participate. The word points out that sometimes doing nothing is itself a choice, and that our silence or cooperation can make us part of something we wouldn't want to be part of.