I guess I've only heard it used to refer to lawyers. Is the term exclusive to lawyers?
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No, it's not restricted to lawyers:
[The American Heritage Dictionary of Idioms by Christine Ammer. S.v. "hang out one's shingle." Retrieved February 8 2016 from http://idioms.thefreedictionary.com/hang+out+one%27s+shingle ]
With respect to the etymology of the phrase, the earliest use I could find with reference to 'hang a shingle' was this from a letter dated January 20, 1830, in The Constellation (also known as the New York Constellation and the Constellation Advertiser), subsequently picked up and republished in the Indiana Palladium, 9 April 1831:
An earlier published instance with reference to writing on a shingle was the following in The Genius of Liberty, 21 February 1829 (Leesburg, Virginia), which was also picked up from a New York paper (unnamed):
A somewhat later publication describing writing on a shingle was this (again) from The Genius of Liberty, 17 November 1832:
Then, in another, later edition of The Genius of Liberty (28 February 1835), this note can be observed:
All together, these occurrences referring to writing on shingles as signage on a bale of cotton (1829), a "lawer's" place-of-business advertisement (1830), and a handy 'slate' for making notes (1832, 1835) tempt the conclusion that shingles were a cheap and ready substitute for more conventional writing surfaces in the early 1800s. The practice of lawyers using shingles to advertise their workplaces was the natural outgrowth of shingles' broader, more general application as signage and 'writing slate'.
Thus, it would be justifiable to suppose that shingles were, early on, used for signage at places of business, regardless of the type of business. To 'hang out a shingle' was equivalent to 'placing a sign'. Additionally, instances of the phrase in early publications invariably specified the exact use of the shingle or, if a place of business was so announced, the type of business that employed the shingle. This broad use of the phrase is further documented in Sven's most excellent answer, with published instances from 1834, 1839, 1843 and 1845.
The earliest Google Books match appears to be from "My Hobby,—rather" in The New Monthly Magazine (November 1834)—and it does involve a lawyer:
It also turns up in this item from the Maumee City [Ohio] Express (October 19, 1839):
But John Bartlett, Dictionary of Americanisms (1848) shows the term already being applied to other occupations as well—and indeed to signboard notices of any kind:
An item in the Edgefield [South Carolina] Advertiser (September 20, 1843) shows it being used in connection with a physician's office:
And an item in the [Ravenna, Ohio] Portage Sentinel (December 24, 1845) shows it being used in connection with a "public dwelling house":
An Ngram chart of "hung out a shingle" (blue line) versus "hang out a shingle" (red line) versus "hung a shingle" (green line) versus "hang a shingle" (yellow line) indicates that "hang out/hung out a shingle" is a somewhat older form than "hang/hung a shingle," and that it remains somewhat more popular today:
However, additional variants on the same idea (such as the 1848 instance of "stuck up their shingles" cited by Bartlett) exist and clearly have been around for a long time.
Maximilian Schele de Vere, Americanisms: The English of the New World (1872) suggests that the expression may have arisen at a time when or in a place where shingles were the standard medium for signage:
Having said all that, I should note that the connection between "hang out a shingle" and lawyers remains particularly strong. Thus, for example, from Edith Shillue, Peace Comes Dropping Slow: Conversations in Northern Ireland (2003):
It may be that the persistence of this idiom in connection with the legal profession is tied to the fact that for many years (in the United States) lawyers were forbidden by law and/or by state bar rules from advertising their services. As a result, the shingle was one of the few acceptable ways of notifying the public of one's availability to provide legal services. This was especially a problem for lawyers setting out on their own practice, rather than joining an established firm.
Hang out one's shingle was originally used especially for lawyers, but is now applied to any kind of profession:
(The American Heritage® Idioms Dictionary)