Back number a person whose methods or ideas are regarded as out of date, and who is no long seen as important. In literal use, this was originally the issue of a periodical before the current one.
back of beyond a term (first recorded in Sir Walter Scott's The Antiquary, 1816, as a humorous phrase for some very out of the way place), used in Australia for the far inland regions remote from large towns or closely settled districts, the backblocks.
back of Bourke the remote and sparsely populated inland of Australia; Bourke is a town in New South Wales.
back-seat driver someone who lectures or criticizes the person who is actually in control, from the idea of a passenger in the back of a car giving the driver unwanted advice.
back slang slang in which words are spoken as though they were spelled backwards (e.g. redraw for warder).
back to basics a political catchphrase of the early 1990s, embodying a conscious return to what are seen as fundamental principles of self-respect, decency, and honesty. The use of the phrase derived from a speech made by John Major, then Prime Minister, to the Conservative Party Conference in 1993.
put someone's back up make someone annoyed or angry, in allusion to the idea of a cat's arching its back in anger.