'atty' originates from English, specifically the word 'attorney', where 'atty' developed as a shortened/abbreviated form of 'attorney'.

'attorney' came into English via Anglo-Norman and Old French (cf. Old French 'atorner' / 'atorne' meaning 'to appoint' or 'one appointed') and later became Middle English 'attorn(e)'/'atorne' before the modern English 'attorney'; 'atty' arose much later as an informal abbreviation of the modern English form.

Initially the related words meant 'one appointed (to act)' or 'to appoint'; over time the meaning narrowed to the modern sense of a legally qualified representative ('attorney'/'lawyer'), and 'atty' simply abbreviates that modern meaning.