The act of hacking someone's avatar account and telling a loved one or friend of theirs that they have changed avatars (aka - are using an alt). The hacker then continues a relationship (using the alt) with the hackee's friend in order to create chaos between them. Thus, a hacksammich is created.
hacker: Hi "Joe"! It's (hackee)! Wanna hang out?
hackee friend Joe: Der....Okay!!
hacker: Excellent! Tell me something about yourself.
Hacker then does the same thing to the hackee, using an "alt" of Joe's. Thereby creating a "hacksammich" and a vehicle to monitor and edit information passed between the two original friends. Chaos ensues.