Where does the expression, "Go scrape mold on yourself" come from? And what does it mean?

My grandfather is from Ireland, and he frequently used this phrase.

I understood it to be an insult, to mean basically "screw you"

I love the phrasing, but have no idea where it comes from. Is anyone else aware of this phrase and knows where it comes from? Or is this a unique saying from my grandfather?

A google search for the phrase "go scrape mould" (using quotation marks, and with the British spelling) returns zero results.

This tells me that it is very unlikely to be a common phrase.

Also, it sounds strange. I would expect something like "go scrape mould off yourself" instead. Usually, we scrape off, not on.