*This term is strictly used within the context of Science Olympiad, a science competition in US.
In case of the most severe scheduling conflicts at competitions, a builder would have to replace an academic backup in a one-person event to form a two-man team to represent the school. The builder, who usually has less than enough knowledge to qualify for the event, is called the head of cabbage.
-- He can't play.
-- I'll be the head of cabbage then.