'Over the left shoulder' definitions:

Definition of 'Over the left shoulder'

From: GCIDE

Left \Left\, a. [OE. left, lift, luft; akin to Fries. leeft, OD. lucht, luft; cf. AS. left (equiv. to L. inanis), lyft[=a]dl palsy; or cf. AS. l[=e]f weak.]
1. Of or pertaining to that side of the body in man on which the muscular action of the limbs is usually weaker than on the other side; -- opposed to right, when used in reference to a part of the body; as, the left hand, or arm; the left ear. Also said of the corresponding side of the lower animals. [1913 Webster]
2. Situated so that the left side of the body is toward it; as, the left side of a deliberative meeting is that to the left of the presiding officer; the left wing of an army is that to the left of the center to one facing an enemy. [Webster 1913 Suppl.]
Left bank of a river, that which is on the left hand of a person whose face is turned downstream.
Left bower. See under 2d Bower.
Left center, the members whose sympathies are, in the main, with the members of the Left, but who do not favor extreme courses, and on occasions vote with the government. They sit between the Center and the extreme Left.
Over the left shoulder, or Over the left, an old but still current colloquialism, or slang expression, used as an aside to indicate insincerity, negation, or disbelief; as, he said it, and it is true, -- over the left. [1913 Webster]