Green’s Dictionary of Slang

(Irish) a disapproving glare, a ‘dirty look’.

2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 16: Old focking Lamb Chop throws me a filthy, basically telling me that I’m pushing my luck.
2005		P. Howard Curious Incident of the Dog in the Nightdress 18: Chloë gives her a filthy.
2021		P Howard Braywatch 24: I notice Currer Bell Whelehan’s mom giving Honor an absolute filthy.