To assist someone, usually in a mischievous or illegal activity. Gary was arrested and charged with aiding and abetting the jewelry thief.I know other kids aided and abetted Paul in egging our house—he's just the only one who got caught.
Cliché to help someone; to incite someone to do something, possibly something that is wrong. (Originally a legal phrase.) He was scolded for aiding and abetting the boys who were fighting.