In politics, someone on the left wing is socialist, a radical liberal. This usage stems from the 1789 French National Assembly. In that body which met on the eve of the French Revolution, the Third Estate, the commoners who where were considerably more radical than the clergy and nobles of the First and Second Estates, were seated on the left side of the chamber.
Use of left to denote liberal (later socialist) views dates to 1837 and Thomas Carlyle’s The French Revolution:
Still less is a Coté Gauche wanting: extreme Left.
Left wing makes its debut in 1884 in William James’s The Will To Believe:
In theology, subjectivism develops as its "left wing" antinomianism.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2016, by David Wilton