Why does Prossy utter the word regarding the costermongers?

Morell often addresses public meetings as part of his political campaign work. His lectures bring him into regular contact with those from the lower classes, whom he wishes to convert to the cause of Christian Socialism. Morell is generally quite enthusiastic about his outreach work, but his secretary Miss Prossy is not so sure. When Morell receives an invitation to address a political organization called the Hoxton Freedom Group, Miss Prossy is scathing, dismissing the group as nothing more than "half a dozen ignorant and conceited costermongers without five shillings between them."
A costermonger is an old-fashioned word for someone who sells fruit and vegetables from a street stall. As always with Shaw, the issue of social class is never far from the surface. Although Miss Prossy is in a more lowly social position than her boss, she's nonetheless something of a snob. Shaw is trying to convince us here that a rigid social hierarchy corrupts everyone, making even the lower classes subscribe to a society that does them no favors.