If you fall overboard, you fall over the side of a boat into the water. His sailing instructor fell overboard and drowned during a lesson
If you throw something overboard, for example an idea or suggestion, you reject it completely. They had thrown their neutrality overboard in the crisis
If you say that someone goes overboard, you mean that they do something to a greater extent than is necessary or reasonable. Women sometimes damage their skin by going overboard with abrasive cleansers
from on board a vessel into the water; "they dropped their garbage overboard" to extremes; "he went overboard to please his in-laws