An Etymological Dictionary of the German Language/flach
flach, adj., ‘flat, shallow, superficial,’ from MidHG. vlach, OHG. flah(hh), adj., ‘flat, smooth’; comp. Du. vlak, ‘even.’ Akin to the graded forms AS. flôc, E. flook, fluke (‘flounder’), North E. flook-footed, ‘flat-footed.’ This suggests Lat. plaga, ‘district,’ or more probably, on account of its meaning, OSlov. plosku, ‘flat’; Lat. plânus scarcely represents *plagnus (see Flur); related to Gr. πλάξ (stem πλακ), ‘surface,’ Gr. πλακοῦς, Lat. placenta, ‘cake.’ But E. flat, OIc. flatr, OHG. flaȥ, ‘flat, level,’ have nothing to do with flach. A MidG. and LG. parallel form of flach is mentioned under Blachfeld.