Human feces used as fertilizer.
In parts of the world where the human population outgrows the nutrients in the soil, night soil can be a big deal. (In some cases people will 'save' their feces and sell them to farmers.) Night soil is especially useful in recycling phosphorus. This is not a new practice; it's estimated that people have been using night soil for almost as long as there's been agriculture. Urine and animal manure are often added, and it is usually composted before use.
The big problem with night soil is that it encourages the spread of parasites, including hookworm, flukes, gastrodiscoides hominis (which causes gastrodiscoidiasis), schistosomiasis, and tapeworm. To avoid transmission of these, night soil should be composted before use, uncomposted night soil should not be allowed to contaminate any water that is going to come into contact with humans, and foods fertilized by night soil should be washed and/or cooked before eating.
Because of the health risk, and the availability of cheap and effective chemical fertilizers, the use of night soil is decreasing.
Sometimes it is written as one word; nightsoil.
Sometimes 'night soil' is just used as a euphemism for feces.