'apace' originates from Middle English, specifically the phrase 'a pas', where 'a' meant 'at' and 'pas' (from Old French) meant 'step' or 'pace'.

'apace' changed from the Old French phrase 'a pas' (literally 'at step/pace') into Middle English as 'apace', and eventually became the modern English adverb 'apace'.

Initially it meant 'at a step' or 'in step' (a spatial or temporal sense), but over time it evolved into its current sense of 'quickly' or 'at a rapid pace'.