'attendingly' ultimately originates from Latin, specifically the verb 'attendere,' where the prefix 'ad-' meant 'to' and 'tendere' meant 'to stretch'.

'attendere' passed into Old French as 'atendre' and into Middle English as 'attenden' (or 'attend'), which became the modern English verb 'attend'; the adverb 'attendingly' was formed later by combining the present-participial/adjectival form 'attending' with the adverbial suffix '-ly'.

Initially, the root sense was 'to stretch toward' or 'to direct' (physically or mentally); over time it evolved into 'to give attention to' or 'to be present with', and 'attendingly' came to mean 'in an attentive manner'.