From English - German Ding/FreeDict dictionary ver. 1.9-fd1:
duller /dˈʌlə/ dümmer, dämlicher, dusseliger, dussliger, doofer, blöder, depperter, törichter, beknackter Synonyms: sillier, more stupid, more foolish, dumber, more dopey, doltisher, more gormless
see: silly, stupid, foolish, dumb, dopey, doltish, air-headed, gormless, dull, silliest, most stupid, most foolish, dumbest, most dopey, doltishest, most gormless, dullest, as thick as they come, thick as a plank, thick as two (short) planks, be (as) thick as two short planks, play dumb, act dumb, Silly me! I've locked myself out of the car.
From English - German Ding/FreeDict dictionary ver. 1.9-fd1:
duller /dˈʌlə/ [Am.] langweiliger, fader, steifer, öder, trüber <adj>