'asunder' originates from Old English, specifically the phrase 'on sundran' and the root 'sundor', where 'sundor' meant 'apart, separately'.

'asunder' changed from Old English 'on sundran' (or related forms using 'sundor') into Middle English forms such as 'asonder'/'asunder', and eventually became the modern English word 'asunder'.

Initially it meant 'apart' or 'separately'; over time the core meaning has remained but the word's use has become more literary or archaic in modern English.