A mass of declassed and excluded individuals separated from the proletariat, which feeds economically on illegal trafficking, the criminal economy, begging, state or private patronage, or a combination of all these.
Proletariat vs. lumpen
The lumpen are not the pre-proletarian "bums and thugs" uprooted from the rural fields and communal lands, they are an uprooted product of the decomposition of bourgeois society.
The lumpenproletariat, that passive product of the rot of the lower layers of the old society, can sometimes be drawn into the movement by a proletarian revolution; however, by virtue of all its living conditions it is rather willing to sell itself to reaction in order to serve its maneuvers.
Karl Marx and Frederick Engels. Communist Manifesto, 1847
Because of their economic activity, often related to organized crime, and their individualism they are closer to the petty bourgeoisie than to the proletariat. Marx ironically points out the mirror-like relationship between the speculative bourgeoisie and the lumpen, given the parasitic and nihilistic character of both classes.
While the financial aristocracy made the laws, ran the administration of the state, had all the organized public powers and dominated public opinion through the de facto situation and through the press, the same prostitution, the same blatant fraud, the same eagerness to get rich, not through production but through the misappropriation of the already created wealth of others, was repeated in all spheres, from the court to the café borgne. And it is remarkable that in the heights of bourgeois society there spread a rampage for the satisfaction of the most unhealthy and disorderly appetites, which at every step clashed with the same laws of the bourgeoisie; a rampage in which, by natural law, the wealth from gambling goes to seek its satisfaction, a rampage by which pleasure becomes crooked and in which money, mud and blood converge. The financial aristocracy, both in its methods of acquisition and in its pleasures, is nothing but the rebirth of the lumpenproletariat at the top of bourgeois society.
Karl Marx. The 18th Brumaire of Louis Bonaparte.
Politically they form a dangerous group, confronted and often instrumentalized against the class movements of the workers. The mere relationship with the lumpen or the attempts to organize it into "gangs" under the authoritarian leadership of some workers' leader, are considered treason from the first moments of the workers' political affirmation.
The lumpenproletariat, that scum made up of demoralized elements from all social strata and concentrated mainly in the big cities, is the worst possible ally. This scum is absolutely venal and most annoying. When the French workers wrote on the walls of their houses during each of the revolutions: "Mort aux voleurs!" Death to the thieves! and indeed shot more than one, they did so not in a fit of enthusiasm for property, but fully aware that first of all it was necessary to get rid of this gang. Every workers' leader who uses elements of the Lumpenproletariat for his personal guard and who relies on them, shows by this fact alone that he is a traitor to the movement.
Frederick Engels. Preface to The Peasant War in Germany.