'attendantly' originates from English, specifically from the adjective 'attendant', ultimately from Latin 'attendere', where 'ad-' meant 'toward' and 'tendere' meant 'to stretch' or 'to extend'.

'attendant' changed from Old French/Anglo-French forms (e.g. 'attendant') and Middle English into the modern English adjective 'attendant', to which the adverbial suffix '-ly' was added to form 'attendantly'.

Initially it referred to 'being present or attending' or 'relating to attending', but over time it has been used to mean 'accompanying' or 'occurring as a consequence', which is the primary modern sense.