Definition of I'm all right jack

I'm All Right Jack is a 1959 British comedy film directed and produced by John and Roy Boulting from a script by Frank Harvey, John Boulting and Alan Hackney based on the novel Private Life by Hackney. The film is a sequel to the Boultings' 1956 film Private's Progress and Ian Carmichael, Dennis Price, Richard Attenborough, Terry-Thomas and Miles Malleson reprise their characters. Peter Sellers played one of his best-known roles, as the trade union shop steward Fred Kite and won a Best Actor Award from the British Academy. The rest of the cast included many well-known British comedy actors of the time.