IPA: /əˈbɛtər/
KK: /əˈbɛtər/
A person who helps or encourages someone to do something wrong or illegal.
The police arrested the abetter who helped plan the robbery.
Abetter is formed from "a-" (meaning to) and "better" (from Old English "betera", meaning superior or improved). The word refers to someone who helps or encourages another to improve or do better.
Think of someone who helps another 'to' become 'better' — that's what abetter means.