A person or thing that broods; a brooding animal, especially a hen that is ready to hatch eggs.
From brood + the agent suffix '-er' (one who does). This follows the productive English pattern of creating nouns for people/things that perform actions.
Farmers used to recognize broomers on sight—a hen that refuses to leave the nest is obsessively committed. Calling someone a broomer was a poetic way to say they were stuck in their own thoughts.
Complete word intelligence in one call. Free tier — 50 lookups/day.