IPA: /əˈbɛtər/
KK: /əˈbɛtər/
A person who helps or encourages someone to do something, especially something illegal or wrong.
The abettor was charged with conspiracy for helping the thief escape.
Abettor is formed from "abet" (from Old French *abeter*, meaning to incite or encourage) and "-or" (meaning a person who). An abettor is a person who encourages or assists someone in committing a crime or wrongdoing.
Think of someone who 'encourages' ('abet') another person to do something wrong — that's why an abettor is someone who helps or incites another.