Green’s Dictionary of Slang

1. in sports, a referee who favours the home team.

1910		Evers & Fullerton Touching Second 188: There are weak umpires and strong ones. The weakest is the "Homer" who gives all close decisions to the home club.
1926		A. Angel ‘Golf Gab’ in AS I:12 627: Baseball has its picturesque allusions to the ‘bunts,’ ‘muffs,’ ‘homers,’ and ‘the mound’.
1955		F. Brookhouser Now I Lay Me Down 118: AB is for Alice, who [...] wound up slugging a homer with the bags loaded.
1960		J. Brosnan Long Season vii: ‘Glossary’ [...] Homer[:] To umpires, an insulting word inferring partiality is being given to the home team.

2. a friend.

1997–2000

Da Bomb 🌐 15: Homer: Used as a generic name. when I greet my friends.