'deathly' originates from Middle English, specifically the word 'deathlich' (or 'deathly'), where the suffix '-lich' (later '-ly') meant 'like' or 'having the quality of'.

'deathly' changed from Middle English 'deathlich' (formed from Old English elements 'dēað' + 'līc') and eventually became the modern English word 'deathly'.

Initially, it meant 'like death' or 'resembling death' (i.e., deadly/like death); over time it also developed the sense of an intensifier meaning 'extremely'.