köken: From Middle High German zǣhe, from Old High German zāhi, zāh, from Proto-West Germanic *tą̄h(ī) (“fitting; clinging; tenacious; tough”). Cognate with Dutch taai (“tough; chewy”), Low German taag (“tough; chewy”), English tough. Doublet of taff.
köken: Inherited from Old East Slavic жестъкъ (žestŭkŭ), from Proto-Slavic *žestъkъ. Cognate to Old Church Slavonic жестъ (žestŭ, “hard”), жестѣти (žestěti, “to harden”). Also related to жесто́кий (žestókij, “cruel”).
köken: From Middle Dutch taey, from Old Dutch *tāi, from earlier *tāhi, from Proto-West Germanic *tą̄h(ī), from Proto-Germanic *tanhuz. Cognate with German zäh, English tough.