I think comrade ZeroNowhere is wrong-- not in his historical information on the phrase.. but in claiming that "you have no right to claim..."
Indeed, if you know what the historical background really is, then you know what the real meaning of Marx's words are-- calling yourself a Marxist is no excuse for being an idiot.... and lazy, i.e. proclaiming your fidelity to "historical materialism" as a reason to not grasp real material history. The real meaning of Marx's words is: "This isn't just about flogging another commodity in the "marketplace" of ideas." Or "Don't fetishize, analyze." Or "Quit whining, and do your homework."