Old slang originally from the county of Kent in England. It refers to an eerie sea mist that forms along the shore which then gradually moves inland bringing with it a chilly, unpleasant clamminess. In his book ‘Great Expectations’ Charles Dickens has Pip meet the convict Magwitch in a thick aggy jagger.
There’s an aggy jaggers come in, can’t see your hand in front of your face.