Foreign secretary Boris Johnson has launched a colourful attack on Labour leader Jeremy Corbyn, describing him as a "mutton-headed old mugwump".

Writing in The Sun, Johnson confirmed the Tories' election strategy to undermine the opposition party by attacking its leader, claiming a Corbyn administration would be a threat to the UK.

By definition, a "mugwump" is someone who is independent when it comes to party politics - but the term actually dates back to a specific point in American history.

The word derives from the Algonquian dialect of Native Americans in Massachusetts and in their language, it meant "war leader".

It became more widely known in the 19th century, during the presidential election in the United States that pitted Grover Cleveland against the Republican candidate James G Blaine.

Jeremy Corbyn
Jeremy Corbyn was labelled a "mugwump" by foreign secretary Boris Johnson Geoff Caddick/AFP

The Mugwumps were Republican political activists who left the Republican Party to support the Democratic candidate Cleveland in 1884, because of the financial corruption associated with Blaine.

Their defection to the Democratic Party reportedly swung the election to Cleveland, by helping him win in New York.

After the election, the term came to mean someone who is independent or who remains undecided or neutral in politics - or someone who distances themselves from party politics.