To regress is to return to a former state or condition, and not usually in a good way. It often means "relapse" or "get worse."
First used in the late 14th century as a noun meaning "act of going back," regress originates from the Latin regress, meaning "a return." Many feel that the regress to anti-social behavior among teenagers is caused by violent video games, TV shows, and movies. A 30 year old forced to move back home may regress to old, childlike habits. It can also happen to someone's health, like when the condition of someone in recovery takes a turn for the worse.