word-forming element meaning "bad, ill, abnormal," from Greek dys-, inseparable prefix "destroying the good sense of a word or increasing its bad sense" [Liddell and Scott], "bad, hard, unlucky," from PIE root (and prefix) *dus- "bad, ill, evil" (cf. Sanskrit dus-, Old Persian duš- "ill," Old English to-, Old High German zur-, Gothic tuz- "un-"), a derivative of *deu- "to lack, be wanting" (cf. Greek dein "to lack, want").
Very productive in ancient Greek, where it could attach even to proper names (e.g. dysparis "unhappy Paris"); its entries take up nine columns in Liddell and Scott. Among the words formed from it were some English might covet: dysouristos "fatally favorable, driven by a too-favorable wind;" dysadelphos "unhappy in one's brothers;" dysagres "unlucky in fishing;" dysantiblepos "hard to look in the face."