What's the meaning of the phrase 'Bish-bosh'?
The reduplicated expression 'bish-bosh' means 'worthless nonsense; trash; foolish talk or opinion'.
What's the origin of the phrase 'Bish-bosh'?
There is another reduplicated phrase bish-bash-bosh, which is indicative of a series of blows or of something done very easily.
At first sight it seems obvious that bish-bosh and bish-bash-bosh are related and that one derived from the other. In fact, that's not the case - the two expressions aren't connected, either in meaning or in origin.
Bish-bosh, with its meaning of 'worthless nonsense', seems to have something in common with 'pish', 'posh' and 'tosh', which all have that same 'nonsense' meaning. Here, there are connections.
Let's look at the words in sequence:
Bosh. This is known since the early 1800s to mean 'nonsense'. The word originated in the Middle East (more below) and is the source of the 'nonsense' meaning of bish-bosh'.
Tosh. Used since the 1890s in the UK to mean 'nonsense'. Usually preceded by an intensifier like complete, utter, awful etc.
Posh. Used in 1920s UK to mean 'nonsense'.
Bish. This began being used to mean 'mistake/blunder' in 1930s UK.
Pish. An 16th century word used as an expression of contempt. Much later, in the 1940s, it began to be used in the UK to mean 'nonsense'.
The term bish-bosh was coined in the USA in the early 19th century. The earliest example that I can find of it in print is from a reported discussion between two lawyers, printed in The San Francisco Examiner, November 1897:
Howe: It will take me less than one day to convince the jury that Martin Thorn is innocent.
Youngs: Bish, bosh. Thorn will be convicted.
Given all of the above dates we can say that the expression 'bish-bosh' pre-dates 'bish' and 'pish' and 'tosh', so none of them can be the root source of the phrase.
As is commonplace with reduplicated phrases, one word supplies the meaning and the second is a rhyming, alliterative or repeated version of the first. In this case the source word is bosh.
Bosh appears to have been coined around 1828. The first printed example of it that I know of is in a 1829 memoir of time spent in Turkey by the Scottish writer Charles MacFarlane - Constantinople in 1828:
If you see an unclean Muscove kill your very child, or your brother in battle, and you should afterwards make him prisoner, you are to put your yataghan [sword] into your girdle... you must not even slit the Karatas ears! Bosh! bosh!
Bosh was inherited from Turkish, where it means 'empty/worthless'.
Why 'osh' words, like bosh, posh and tosh all came to be selected to mean nonsense isn't clear.