der Hochstapler — the impostor. Duden: a person who pretends to a high social position in betrügerischer Absicht, with intent to deceive. The word comes from thieves’ cant, from hoch, genteel, and stapeln, to beg.
tiefstapeln — to stack low. To understate one’s own abilities.
der Scheinriese — the apparent giant. Duden: an overrated person or thing deren Bedeutung bei näherer Betrachtung geringer erscheint, whose importance seems smaller on closer inspection. The word comes from a figure in Michael Ende’s children’s book Jim Knopf und Lukas der Lokomotivführer. Herr Tur Tur looks huge from far away and smaller the closer one comes.
gefährliches Halbwissen — dangerous half-knowledge. Said with irony of a little knowledge presented with confidence.