Green’s Dictionary of Slang

grot n.

[grotty adj.; note N.Z. school/institutional use grot, lavatory]

1. a dirty, untidy person.

[US]Baker et al. CUSS 130: Grot An obnoxious person.
[UK]A. Warner Sopranos 120: ‘It’s Henry!’ [...] ‘It’s a fucking ancient old ugly grot that’s what’.

2. dirt, detritus.

[UK]‘Derek Raymond’ He Died with His Eyes Open 123: You stink with the grot of that Staniland case.
[UK]K. Lette Llama Parlour 47: Instead of an orgy of grot and grime she led me into a spotlessly clean room.
[UK]Guardian G2 21 June 12: This higgeldy-piggeldy mix of grace and grot.
[Aus]S. Maloney Sucked In 161: He wore [...] a grot-marinated gabardine raincoat.
[Scot](con. 1980s) I. Welsh Skagboys 113: The hotel [...] wis an Algerian grot-hole.
[Aus]C. Hammer Silver [ebook] Compared to his Corolla, it’s grot-free.

In derivatives

grotbag (n.)

a repellent individual.

[Scot]I. Welsh Dead Man’s Trousers [40]: I was almost even tempted to call Syme to pull a favour, but I’m staying away from that grotbag.