E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
On the shady side of fortythe wrong side, meaning more than forty. As evening approaches the shadows lengthen, and as man advances towards the evening of life he approaches the shady side thereof. As the beauty of the day is gone when the sun declines, the word shady means inferior, bad, etc.; as, a shady character, one that will not bear the light; a shady transaction, etc.