from Wiktionary, Creative Commons Attribution/Share-Alike License
- People of similar character, background, or taste tend to congregate or associate with one another.
The expression appears to have surfaced in the 16th century, allegedly a literal translation of Plato's Republic. In 1545, William Turner wrote a version of the expression in the Rescuing of Romish Fox: "Byrdes of on kynde and color flok and flye allwayes together." (Wiktionary)