Emily Dickinson Lexicon

LEFT, a. [L. lævus; Gr. λαιος, Hesych. λαφος; probably from the root of leave, Gr. λειπω, and properly weak, deficient. Applied to the hand or arm, it denotes the weak arm, as opposed to the right, the strong or dextrous. Hence the ancient idea of sinister, unfortunate, attached to the left arm or side.]

1. Denoting the part opposed to the right of the body; as, the left hand, arm or side. Hence, the noun being omitted, we say, on the left, that is, on the left side or wing, as of an army.
2. The left bank of a river, is that which is on the left hand of a person whose face is toward the mouth of the river.