A slightly posh reference to canine faecal matter. Used by Mums to describe dog shit.
What's that smell, have you trod in dog dirt?
If I could do any job for a day I'd go round and clean up all the dog dirt (a geniune response to a phone in on radio Leicester; 'Describe your ideal job for a day'! Excellent, dripping with post-menopausal outrage... like anyone was stopping her doing it anyway!)