'attn' originates from English, specifically the word 'attention', which ultimately comes from Latin 'attentio' (from 'attendere'), where 'ad-' meant 'to/toward' and 'tendere' meant 'to stretch'.

'attention' changed from Old French 'atencion' (from Latin 'attentio') and eventually became the modern English word 'attention'; 'attn' developed later as a clipped written abbreviation of 'attention'.

Initially, it meant 'the act of attending or stretching toward something' (from Latin sense), then came to mean 'mental focus or notice'; in modern usage a clipped form 'attn' is used specifically to mark recipients or direct notice on documents.