Subposting is sort of like subtweeting but on any platform of social media. Most people subpost with the intention of secretly talking about or referencing (typically a person) that they are angry or annoyed at without directly naming or tagging said person(s). Sometimes subposting can be in a positive manner but usually not as it is an activity usually used by passive-aggressive people.

When you eat dirt for the first time and leave your family to build a shack in the woods