atrocious: [17] Traced back to its ultimate source, atrocious meant something not too dissimilar to ‘having a black eye’. Latin āter was ‘black, dark’ (it occurs also in English atrabilious ‘melancholic’ [17] – Greek mélās meant ‘black’), and the stem *-oc-, *-ox meant ‘looking, appearing’ (Latin oculus ‘eye’ and ferox ‘fierce’ – based on ferus ‘wild’, and source of English ferocious – were formed from it, and it goes back to an earlier Indo-European base which also produced Greek ōps ‘eye’ and English eye).
Combined, they formed atrox, literally ‘of a dark or threatening appearance’, hence ‘gloomy, cruel’. English borrowed it (in the stem form atrōci-) originally in the sense ‘wantonly cruel’. => eye, ferocious, inoculate, ocular
atrocious (adj.)
1660s, from stem of Latin atrox "fierce, savage, cruel" (see atrocity) + -ous. Colloquial sense "very bad" is late 19c. Related: Atrociously; atrociousness.