What line are you in? What trade or profession are you of? “In the book line” —i.e. the book trade. This is a Scripture phrase. “The lines have fallen to me in pleasant places, yea, I have a goodly heritage.” The allusion is to drawing a line to mark out the lot of each tribe, hence line became the synonym of lot, and lot means position or destiny; and hence a calling, trade, or profession. Commercial travellers use the word frequently to signify the sort of goods which they have to dispose of; as, one travels “in the hardware line,” another “in the drapery line,” or “grocery line,” etc.
(The). The equator. (See Crossing The Line.)
The deep-sea line. A long line marked at every five fathoms, for sounding the depth of the sea. The line. All regiments of infantry except the foot-guards, the rifle brigade, the marines, the militia, and the volunteers.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894