Большой русско-английский фразеологический словарь
М136
НА РЫБЬЕМ МЕХУ coll, humorPrepPInvarnonagreeing postmodif or subj-compl with быть0 ( subj: a noun denoting an item of outer clothing)fixed WO
Смотреть другие описания( sth. is) of no use to a person against the cold, of no warmth, often because it is worn-out: у X-a пальто на рыбьем меху - X's coat has no warmth to itX's coat offers no (little) protection from the elementsX's coat is no protection from the coldX's coat is threadbare.