Idealistic bourgeois dissidents idealised the proletariat as entirely made of pure, gentle manly men and brave women. When confronted with the reality that capitalist misery + hetero patriarchy created alcoholic domestic abusers they dismissed them as not true proletariat, hence the term lumpen.
How can I begin to describe how unserious this is
Edit : Apparently I misunderstood or I had a bad source, turns out the term was actually coined by Marx and Engels. Idealisation of the proletariat is still a real thing but it's not the origin of the lumpen term