sewer rat

plural sewer rats

1. The brown rat (Rattus norvegicus).

   1 more example

   The black rat [...] became extinct in Britain following the Roman withdrawal, only to reappear in Norman times. Thereafter the black rat prospered - until the arrival of the larger brown rat (also known as the sewer rat or Norway rat), which reached Britain at about the time of the Hanoverian succession in the eighteenth century.

   2018, Tim Flannery, Europe: A Natural History, page 272

   Synonyms: Norway rat, Norwegian rat

2. derogatoryA contemptible person.

   Synonyms: scumbag

3. Used other than figuratively or idiomatically: Any rat that lives in sewers.