Green’s Dictionary of Slang

(Aus.) a general intensifier, e.g. dirt mean.

[1849		Liverpool Mail 10 Mar. 3/3: ‘An what might your charge be?’ [...] ‘Only a matter of a shilling!’ ‘Well, that’s dirt cheap’].
1911		E. Dyson ‘The Picnic’ in Benno and Some of the Push 9: He’s too dirt mean to finance a skirt iv ’is own.