unlucky, Right lucky. The augur among the Romans having taken his stand on the Capitoline Hill, and marked out with his wand the space of the heavens to be the field of observation, divided the space into two from top to bottom. If the birds appeared on the left side of the division, the augury was unlucky, but if the birds appeared on the right side the augury was pronounced to be favourable.
“Hail, gentle bird, turn thy wings and fly on my right hand” but the bird flew on the left side. Then the cat grew very heavy, for he knew the omen to be unlucky. —Reynard the Fox, iii.
The Left, in the Legislative Assembly of France, meant the Girondists; it was famous for its orators. In the House of Commons the Opposition occupies the left-hand side of the Speaker. In the Austrian Assembly the democratic party is called The Left.
Over the left. A way of expressing disbelief, incredulity, or a negative. The allusion is to morganatic marriages (q.v.). When a woman so married claimed to be a wedded wife, she was told that such was the case “over the left.” (See below.)
Sinister (the left hand), meaning not straightforward, dishonest, is far older than morganatic marriages. The ancient Greek augurs considered all signs seen by them over the left shoulder to be unlucky, and foreboding evil to come Plutarch, following Plato and Aristotle, gives as the reason; that the west (or left side of the augur) was towards the setting or departing sun.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894