What's the meaning of the phrase 'Night soil'?
Euphemism for human faeces.
What's the origin of the phrase 'Night soil'?
In many cities in the developed world human wastes used to collected from storage tanks called privy vaults. The wastes were called "night soil" and were sold to farms as fertilizer. This practice continued until early in the twentieth century but the term was coined much earlier. So early in fact that the first known example of the expression in print comes from a newspaper that ceased publication in 1735 - the English paper The Daily Courant. Here's a piece from an edition in May 1721:
One Bentley was by Order of the Commissioners of Sewers for the said City and Liberty of Westminster ... prosecuted for shooting Night Soil into a Grate over the Common Sewer in the Broad-Way in the Parish of St Giles in the Fields; and being thereof Convicted, was sentenced to be whipt from White-Hart Corner in the said Parish to St Giles’s Pound, which sentence was accordingly executed upon him.