'anon' originates from Old English, specifically the phrase 'on an' (also written 'on ān'), where 'an' meant 'one'.

'anon' changed from Old English 'on an' into the Middle English form 'anon' and eventually became the modern English word 'anon'.

Initially it meant 'in one' or 'at once' (immediately), but over time it evolved into the literary/archaic sense 'soon' and the modern slang sense 'anonymous'.