The word sinister (sinistro) in Italian has several meanings, all very well-known, and among them it comes to indicate accidents, and always something unfortunate, unpleasant.
But sinister in Italian refers primarily to what, facing north, is to the west side: “left”. So it refers to the hand that is mostly weaker, because it is less frequently exercised by the generality of human beings, that in ninety percent of cases are right-handed, and only the remaining (7-10) left-handed.
Left-handed in English has no mystery, but in Italian we use the word “mancino”, a strange word that comes by Latin mancus, meaning “maimed”, “crippled”, and so that: “having lost his good hand, must necessarily get used to act with the other one”.
English language uses left, and that word originally had the same sense as the Latin mancus, of “weak”, “useless”, (see etymonline.com: c.1200, from Kentish and northern English form of Old English lyft- “weak, foolish”).
But the Latin root is present in English with sinister, that has no “physical-geographical” value, but it carries only “unfortunate”, and “ominous” meanings. So the English speakers usually ignore that there is a relation between “left” and “sinister”.
In Spanish (similarly Portuguese) the left is called izquierda, and it is independent from the Latin word, and probably derived from Basque-Celtic, and perhaps esku in Euskera “hand” joined to the Celtic kerros which means “bent”, “crooked”. In Basque eskerre means “left” and is also “clumsy hand”.
But also the Spanish has a word derived from the Latin sinistrum: siniestro, with a range of meanings that is modeled on the Italian, including the accidents (car accidents and insurance related procedures).
The etymological dictionary of Giacomo Devoto reports: the Latin sinister, derived from sinis, (and composed as magister from magis) *Sinis means “unlike” united to -ter root for comparison and contrast. Sinister indicates what is “different from normal”, and, in particular “the abnormal (hand)”. In Latin, sinister took the place of the two oldest words: laevus & scaevus (from the second comes the famous Gaius Mucius Scaevola who burned his hand… see if interested: http://en.wikipedia.org/wiki/Gaius_Mucius_Scaevola). In general, in the Indo-European lexicon, the terminology “right” is stable (because it is the “normal” side), while the “left” is the strange one. This is not to say that only the right brings good luck, because luck can also be associated with abnormality, extraordinary things.
The website etimo.it shows some interesting data on the origin of the word “left”.
Someone leads it back to sinus, “breast”, that is which is “hidden under the folds of the dress”, just as with the Roman toga and the left hand. Others lead it back to senex “old” as saying: “more worthy” and terminating in -ter as magister, minister.
The dictionary goes on quoting as in Rome (ancient) the word was also used with the opposite meaning to the current one (and with the sense of: happy omen, prosperous, joyful) as in religious rites the Romans were facing south, and then the good wishes came from the left, cause the east was there (and there is from which the sun arises).
In other cultures, however, including the Greek, who prevailed, rites were officiated facing north, and so the left, the weak hand, was in the west, where inauspicious omens come. From there went on the sedimentation of the current meaning in our culture, which among other things, and besides the orientation of rites and temples, has its own maps and charts oriented north and the west to the left.
Clearly, the combination of sunrise light and the east, with birth, positivity, good luck, life, good, and the opposite, the setting sun, with death, disgrace, darkness and end, etc. is completely natural and understandable.
All this, together with the fact that usually the human being (in a rare occurrence among the species of the planet) has a majority predisposition for the use of one body part, and that the less used is the left, weaker, therefore, insecure, incompetent, gave rise not only to the wide range of meanings that the word “left” (and especially “sinister”) plays in many Western languages, but also to a number of prejudices about left-handedness.
It should be noted how rare are the cultures that have not given a symbolic value to the use of the left hand (China, for example, had no prejudice against lefties), while in others, it has even arrived to the excess of discrimination against the left-handed, forcing their correction (which today, it has been clarified, is insane and the origin of neurological problems), as well as a series of ridiculous precepts and religious requirements. About them, for stupidity, (what a surprise!) Christians and Muslims shine.
Even this story can be used as an example to illustrate human stupidity, the blindness aggressive intimidation of religions and especially of certain ones.
Today left-handedness is something absolutely peaceful, it is even considered to be a good thing in some ways. It seems that even a greater number of left-handed people have had special skills and success. The Internet is full of websites that show lists of famous lefties as for example Einstein, Napoleon, Obama, Bill Gates, Charles the Great, Julius Caesar, and so on.