A cheeky compliment for something outrageously good, especially a bit of football skill like a nutmeg, screamer, or silky pass. If it’s so smooth it feels unfair, it’s filthy. The joke is it sounds nasty, but it’s pure praise, the kind you shout when someone’s just embarrassed a defender and made it look easy.
"Did you see that nutmeg at five-a-side? Filthy. He sat the lad down, took two touches, then fizzed it top bins before anyone could blink."