An Etymological Dictionary of the German Language/dumpf
dumpf, adj., ‘damp, dull, heavy,’ ModHG. only; formed by the weakest stage of gradation from MidHG. dimpfen, str. vb., ‘to fume, smoke’; comp. also MidHG. dumpfen, dümpfen, ‘to fume, damp.’ The orig. sense of the adj. is probably ‘smoky,’ i.e. ‘damp,’ or ‘dimming the sight and dulling the hearing’; dumpf appears in Du. dompig, with the meaning ‘damp, gloomy.’ Perhaps the word is connected with dunkel; comp. E. dank.