These words—“everybody” and “nobody”—are indefinite pronouns, meaning they don’t refer to a particular person. Both these indefinite pronouns are singular. This is important information, as you need to know if the subject is plural or singular in order to use the correct verb form. This sentence is incorrect:
Everybody know the chickens are fed by noon.
This is correct as the subject and verb agree:
Everybody knows the chickens are fed at noon.
Indefinite pronouns can be tricky. Some are singular, such as “everybody” and “nobody,” as well as “nothing” and “something.” Some are plural, such as “few” and “many.” Still others can go either way depending upon usage.
There’s no easy way to identify whether an indefinite article is singular, plural or variable just by looking at it. You’ll need to consult a dictionary and commit the information to memory. Many grammar books have a handy list that you might keep for reference.