An Etymological Dictionary of the German Language/Stirn
Stirn, f., ‘forehead, brow,’ from the equiv. MidHG. stirne, OHG. stirna (for *sternja), f.; a specifically HG. word (yet also in AS. steornêde, ‘frontosus’?), for which Du. voorhoofd, AS. foranheáfod, E. forehead (OIc. enne, Goth. *anþi, OHG. ęndi, equiv. to Lat. antiae), occur. In Bav., Hirn is generally used instead of Stirn. The form *sternjô- has been connected with Gr. στέρνον, ‘breast,’ while ‘broad’ is assumed to be the intermediate idea, which is deduced from the root ster, in Lat. sternere and Gr. στρώννυμι, ‘to spread out’; comp. OSlov. strana, ‘district.’