Why is talking about work called "shoptalk"?

Why is it called "talking shop" when two colleagues talk about their work or business in a social situation, especially in a way that's boring for others? I don't know what the word "shop" has to do with, for example, the situation in which two farmers talk about their crops at a family party. I mean why is it not "talking business", for example, which is a more general term?

Solution 1:

A shop is (from OED)

2. A house or building where goods are (i) made or (ii) prepared for sale and also sold., (e.g. “an engineering shop”; a sweatshop)

3.a. A building, room, or other establishment used for the retail sale of merchandise or services. e.g. "A shoe shop"; "a grocery shop".

Shop then extended in meaning to the general:

6.a. colloquial or slang. A place of business, an office; the place where one works. Now rare.

1776 D. Garrick Let. 14 May (1963) III. 1098 I cannot, till I make my transfer, be absent from the Shop [sc. a theatre] one Night.

1827 T. S. Surr Richmond II. i. 5 I hurried off with Bucks to the office, or shop, as he called it.

It then widened further:

7. (Discussion of) matters relating to one's trade or profession, esp. when introduced inappropriately into general conversation. Chiefly in to talk shop at Phrases 12.

1814 Last Act i. iii, in J. Galt New Brit. Theatre II. 379 Come, Tom, no shop now.

To talk shop (the same construction to "to talk French") is first recorded by the OED:

P12. to talk shop: to talk about matters relating to one's own business or profession, esp. when it is not appropriate to do so.

1833 Age 5 May 143/2 And so are you all honourable asses, to be talking ‘shop’, after this fashion, while the grog and the Venusses are waiting.

The development of the phrasal verb is therefore quite natural.