Q From Ann Deane, UK: I wonder if you have heard a phrase that my grandmother used with regularity (she died in 1964). “There have never been such times since old leather-arse died”. She would use it whenever she was told about something new — if decimalisation was suggested, or television was getting another channel, or when we told her a sputnik had gone up.
A The expression is hardly known now except among older people. Nigel Rees collected a number of versions from listeners to his radio programme in his book Oops, Pardon, Mrs Arden! Among them was, “I haven’t laughed so much since old leather arse died!” Much the same term also appears in Nancy Keesing’s book Lily on the Dustbin: Slang of Australian Women and Families, published in 1982: “A term of approval meaning ‘very good indeed’ in response to the question, ‘What do you think of ...?’ is ‘Damn the better since Leather Arse died.’”
Leather arse is also known by itself, as a self-deprecatory slang term among motorbike riders and horsemen. Tam Dalyell records it as US poker slang — in the form leather ass — for someone exhibiting great patience, which also appeared (in its British spelling) in a 2001 Daily Telegraph article about being successful at poker: “‘The qualities you need to succeed are aggression, timing, intuition and perhaps, above all, patience,’ [Al] Alvarez says. ‘You’ve got to develop what the Texans call “a leather arse”.’” It may at one time have been British services slang, to judge from one reference I’ve found. There’s also the politer or bowdlerised version leather-bottoms, which Eric Partridge and Jonathon Green record in their slang dictionaries as a collective term for civil servants who are so dedicated to their work they never leave their desks.
But nothing suggests who this personified Leather Arse might have been. One posting online suggests that it might have originally been Irish and referred to Oliver Cromwell, who allegedly used to wear leather breeches. But as the writer says, Cromwell is blamed for a lot of things in Ireland and the story is probably just a popular etymology.
So, while I’ve confirmed your grandmother was using an expression that once was widely known, I can’t help with where it comes from.