rotten borough

Meanings

1. A parliamentary borough that was represented in Parliament although the number of voters had diminished so greatly that they were largely controlled by the main landowner; such boroughs were abolished in the 19th century.
2. A parliamentary constituency or electoral district in a similar situation.

Etymology

From rotten (“in a state of decay”) + borough (“type of administrative district”), because such boroughs were regarded as having “decayed” due to their voters moving away to other places.