'pa' originates from English, specifically the informal shortening 'papa', where the element 'pa' is a child-formed term meaning 'father'.

'pa' changed from the earlier child-word 'papa' (used in English and French), which itself comes from Latin 'papa' and Greek 'pappas', and eventually condensed in colloquial English to 'pa'.

Initially, it meant 'father' in baby-talk/child language, and over time it has remained in modern English as an informal term for 'father' (dad).