An Etymological Dictionary of the German Language/Annotated/steil
steil, adjective, ‘steep,’ from the equivalent late the variants of which, steigel, Old High German steigal, prove the origin of steil (literally ‘mounting’) from the cognates discussed under Steig. Compare Dutch steil, Anglo-Saxon stœ̂gl, stœ̂ger, ‘steep’; to these are allied Old High German stëcchal, stëhhal, Middle High German stëckel (stëchel), ‘steep,’ Bavarian, Alsatian, Middle High German, and Low German štickel, ‘steep’ (in the Upper German dialects steil seems to be entirely unknown).