Belabor means to go at something with everything you've got. When you say, "Don't belabor or agonize over the decision," it means, "Move on."
Belabor is made up of the Latin roots be and labor meaning "to exert one's strength upon." You can belabor a point by using excessive detail, or you could belabor the obvious by stating over and over what everyone already knows. Belaboring can be a physical attack as well. A person can belabor or beat the living daylights out of you with a club.
