Apparently, Louis Brandeis, Associate Justice on the Supreme Court of the United States from 1916 to 1939 coined the phrase, “sunlight is said to be the best of disinfectants.” A 1913 Harper’s Weekly article, entitled “What Publicity Can Do,” is where you can find the original article. (Hat tip to the Sunlight Foundation.)
So is it true? No.
While sunshine is a very good disinfectant, but it takes longer to disinfect:
And it might depend on what you are disinfecting: