(1):

A line pert. or attached to the side of a thing.

(2):

Specif., a line for hobbling an animal by connecting the fore and the hind feet of the same side.

(3):

A line of goods sold in addition to one's principal articles of trade; a course of business pursued aside from one's regular occupation.

(4):

A secondary road; esp., a byroad at right angles to a main road.