E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
What line are you in? What trade or profession are you of? In the book linei.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.