To be dumb is to have a substantially inferior thinking ability as compared to the regular norm. It is not a recognized medical term, but is a part of everyday speech. There are also temporary choices or decisions that are made that are unwise that could be called dumb even if the person making them generally does not suffer from a diminished thinking ability.
A second meaning of "dumb" is an inability to speak. This is a more archaic term, but is common in older literature, including the Bible. A more common word for dumb today is "mute". During his earthly ministry, Jesus healed the "dumb". An inability to speak could be a condition from birth or else manifest itself later in life through an accident or illness. Strokes have been known to render their victims unable to speak, although oftentimes this is temporary.