An Etymological Dictionary of the German Language/garstig
garstig, adj., ‘filthy, foul, obscene,’ an extended form of the late MidHG. garst, adj., ‘rancid, tasting “high”’; comp. Du. garstig, ‘insipid, rank, rotten’; akin to OIc. gerstr, ‘morose’ (in appearance). Allied to Lat. fastidium, ‘disgust, aversion’?. The latter probably represented *farstidium, like tostus for *torstus, from torreo; Lat. f initially corresponds to Teut. g. See under Galle (Lat. fel). But it might perhaps be also connected with Lat. horridus for *ghorsidus.