Brewer's: Samian Letter
(The). The letter Y, used by Pythagoras as an emblem of the straight narrow path of virtue, which is one, but, if once deviated from, the farther the lines are extended the wider becomes the breach.
When reason doubtful, like the Samian letter, Points him two ways, the narrower the better.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894