Ironic, mood-lightening expression to be used upon bearing witness to an unpleasant shitstain or skidmark.
A prison guard examines an inmate's bed, peeling back the covers to reveal a large brown stain of fecal matter plastered over the sheets. To bring humour and gravitas to the discovery he remarks, "That's not gravy." (As seen on the television documentary 'Strangeways'.)
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