'co-directional' originates from a combination of the prefix 'co-' (from Latin 'com-') and the noun 'direction' (from Latin 'directio'), where 'com-' meant 'together/with' and 'directio' meant 'a guiding or straightening'.

'direction' comes from Latin 'directio' (from 'dirigere' meaning 'to guide/straighten'), passed into Old French as 'direction' and into Middle English as 'direction'; the prefix 'co-' (from Latin 'com-') was attached in Modern English to form 'co-directional' meaning 'sharing a direction'.

Initially the elements meant 'together' + 'guidance/line', and over time the combined form came to mean specifically 'having the same direction' in modern usage.