Green’s Dictionary of Slang

shitter n.³

a coward.

1996		P. Howard The Joy (2015) [ebook] ‘Where are youse going, yiz shiters?’.
2001		I. Welsh Glue 55: Maist guard dogs are shiters. Thir bark’s worse thin thir bite.
2005		P. Howard Curious Incident of the Dog in the Nightdress 282: I starts walkin’ away from him and says he, ‘Shitter’.