Left has gotten a bad rap throughout history. Because of overwhelming majority of people are right-handed (most estimates are in the range of 85 to 90 percent), left-handedness has come to be associated with weakness — the word left itself is descended from an Old English word meaning “weak.”
Left-handedness was therefore until recently often seen as undesirable, and even well into the twentieth century, parents and teachers often forced left-handed children to use their right hand for writing, eating, and other basic activities. Even now, “a left-handed compliment” (also described as “a backhanded compliment”), refers to an ostensibly positive comment that is explicitly or implicitly an insult.
Idioms that employ left to describe an undesirable or unusual situation include “two left feet,” referring to a clumsy dancer, and “out of left field,” meaning an unexpected comment or idea. (The latter, however, is not necessarily derogatory.) We also use left to refer to something that remains behind as a result of deliberate action or accidental oversight. Another common idiom with a negative connotation, one using this sense of left, is “left a lot to be desired.”
The equivalents of left in other languages have similarly pejorative meanings. Gauche, the French word for left, also means “tactless, crude, socially inept” — in English as well as French. The opposite, droit, is the root word of maladroit, which means “incompetent, inept, unsuitable.” (English has adopted and adapted that term as adroit — literally, “to the right,” and meaning “appropriate” — as well as maladroit.)
Sinister, from the Latin word for “on the left,” came to be associated with inauspicious or unlucky events, and was borrowed by French and later English to mean “evil.” In heraldry, it refers to the right-hand side of a coat of arms (the left-hand side from the point of view of the bearer of a shield, from which the coat of arms derived), opposite the dexter, or right, side. From the Latin element dextr-, meaning “on the right,” borrowed into English as dexter, we also get the adjective dexterous, meaning “clever, skillful.”
Right itself means “good, correct,” and that’s the originally connotation when referring to the right hand — it’s the correct one to use. Among the many idioms suggesting the positive connotation are “right-hand man” and “the right stuff.” (The use of right and leftto refer to political ideology, each often capitalized when referring to adherents as a collective, came from the revolutionary era in France: The conservative party in the National Assembly called itself the Droit, the “right” party. The liberal faction, in opposition, came to be referred to as the “left.”)