On, towards, or relating to the side of a human body or of a thing which is to the west when the person or thing is facing north. The original meaning (in Old English) was ‘weak’, the left-hand side being regarded as the weaker side of the body.
Left also means of or relating to a person or group favouring radical, reforming, or socialist views. The usage is first recorded in English in Carlyle's account (in The French Revolution, 1837), and originated in the French National Assembly of 1789, in which the nobles were seated on the President's right and the commons on his left; this ceremonial grouping soon came to reflect political views.
Left Bank a district of the city of Paris, situated on the left bank of the River Seine, to the south of the river, an area noted for its intellectual and artistic life.
left field in baseball, the part of the outfield to the left of the batter when facing the pitcher; in extended usage, it is a surprising or unconventional position or style (as in out of left field), or a position of ignorance, error, or confusion.
left-handed marriage a morganatic marriage (it was customary, in such marriages in Germany, for the bridegroom to give the bride his left hand instead of his right).
See also the right hand doesn't know what the left hand's doing.