'muff' being another word for vagina.
one who inserts fingers into the vagina, or 'muff' if you will; and therefore is digging a muff.
"'ow woz all da muff digging ya muff digger?"
"how was all that vagina digging, you vagina digger?"
Similar to a gold digger. One who dates sluts so they can get some muff.
Steve likes slutty girls, hes always trying to get some. What a muff digger.