from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Slang A horseracing enthusiast, especially one who watches races at the outer rail of the track.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A rail or similar bird
- n. A gambler; originally specifically a horseracing enthusiast
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a fan of racing who watches races from the outer rail of the track
rail + bird (Wiktionary)