A 'swall rat' is someone who constantly is drinking alcohol, their 'swall.' A Northern Ireland slang word, used amongst younger people. When people call others a 'swall rat' it is usually taking the piss but can be used seriously. Also see 'swall demon.'
1. Oh my word, you had one beer? You swall rat!
2. She's a complete swall rat, she was just swallin' on her swall all night long.