"Dim memory" refers to a vague or unclear recollection of something. It means that the memory is not very vivid or detailed, and may be difficult to recall or picture in the mind. Full definition
He'd seen pictures of his father before and had a few dim memories of a man who laughed and smiled and didn't drink most every hour of every day. (bookbrowse.com)
As baseball's new season begins, the previous one recedes into dim memory, even for the World Series champions (si.com)
But with determination and inventiveness, perhaps one day the environmental consequences of the private car will be as dim a memory as the horse pollution crisis of the 19th century. (evworld.com)