An Etymological Dictionary of the German Language/siech
siech, adj., ‘sickly, infirm,’ from MidHG. siech, OHG. sioh (hh), adj., ‘sick’; corresponding to OSax. siok, Du. ziek, AS. seóc, E. sick, OIc. sjúkr, Goth. siuks, ‘sick.’ The OTeut. term for ‘sick, ill,’ compared with the ModG. word krank; comp. the difference in meaning between ModHG. siech and Seuche. To this Sucht is allied (and schwach?), as well as Goth. siukan, str. vb., ‘to be weak.’ A pre-Teut. root sug is wanting.