Preceded by the definite article the, breadline denotes the poorest condition in which it is acceptable to live—synonym: subsistence level.
The literal meaning of breadline (originally bread line) is a queue of poor people waiting to receive bread or other food given as charity. The earliest instance that I have found is from the Democrat and Chronicle (Rochester, New York) of Monday 4th April 1898:
At the Labor Lyceum, yesterday afternoon, the usual 3 o’clock meeting was held under the auspices of the Prohibition Union of Christian Men. C. N. Howard, president of the union, delivered an address […].
Mr. Howard went on to say that there was no occasion for so much poverty and suffering, and described the great banquet given some time ago in New York city when a hundred guests sat down to a banquet that cost a hundred dollars per plate; and told of the bread line at Fleischmann’s bakery, on Broadway, where, on the same evening, 479 men, by actual count, were waiting their turn to get the charity ration of stale bread doled out there to the poverty stricken.
Bowery men in bread line at Fleischman’s Restaurant, N.Y.C. – Thursday 2nd January 1908
source: Library of Congress, Prints & Photographs Division
Originally therefore, bread line denoted the queue of needy men waiting to be given bread outside Fleischmann’s Vienna Model Bakery, on the corner of Broadway and 10th Street, New York City. The following is the obituary of Louis Fleischmann (1836-1904), published in The Gazette (Montreal, Quebec) of 26th September 1904—the name has also been spelt Fleishmann:
New York, September 25.—Louis Fleishmann, the millionaire baker and philanthropist, died here early today at his home in West 77th street, of paralysis. Mr. Fleishmann was born in 1836 near Olmutz, Moravia. He fought in the war of 1866 against the Prussians and won distinction in the battle of Sadowa. He remained in the army until 1874, when he resigned to emigrate to America. He opened the Vienna Model Bakery, at the Centennial Exposition and later transferred it to this city, and at Christmas, in 1876, he established the unique charity known as “The bread line,” and ever since has distributed unsold bread nightly to all who have applied. The “Bread Line” of applicants grew until as many as 5,000 loaves a night were handed out, and in the winter cups of coffee were given with the bread. Mr. Fleishmann also established an employment bureau, went personally among the unfortunates in his “Bread Line” night after night, and found work for many men. His wife, three sons and two daughters survive him.
I have found an early figurative use of bread line in The Labor World (Duluth, Minnesota) of Saturday 24th August 1907; reporting on a strike conducted by the local telegraph operators, this newspaper denounced the telegraph magnates as
perfectly satisfied to keep the wages of the telegraphers close to the bread line, while they revel in luxury from unlimited profits.