In fantasy football, a "lame duck" refers to a team manager who is extremely inactive. This person is a detriment to the league because they do not set their lineups every week. This leads to injured players being started, minimal trade activity, and an overall lack of care with respect to how their team performs.
Lame duck managers are a detriment to fantasy football leagues because matching up against a lame duck-owned team is usually a guaranteed win. This throws off the competitive balance of a league and gives some teams an unfair advantage.
The term "lame duck" originally applies in sports to coaches who are about to lose their jobs at the end of the season and, therefore, don't have an incentive to be successful. These coaches are often less important and less effective.
Lame duck owners often surface because their teams are not good enough to compete and they give up. Many lame duck owners also stop setting their lineups because they forget or the league is not important enough to them.
The best way to avoid having lame duck owners is to control who is allowed in the league from the beginning. It is the responsibility of the commissioner to choose participants who will actually try to be competitive, even if the season is not going their way.
If a lame duck owner does emerge during the course of a season, though, the league commissioner can usually usurp power and take over the neglected team, depending on the league rules. Another option is to try and find another person willing to take over for the lame duck owner.