Concept of Earthing
Earth is a conductor, and acts as sink or source of charge i.e. it can provide any amount of charge as per need. i.e. Whenever any charged body is earthed, due to Coulomb's attraction between charges of opposite polarity, it extracts the charge of opposite polarity from earth, so that it becomes neutral.

Charge on given body As the electrons(i.e. negative charge) flows from the earth into the body. That means, the body must have been positively charged(opposite polarity) initially.