'ad' (short for 'advertisement') originates from English as a clipped form of 'advertisement', which comes via Old French and Middle English from Latin 'advertere' meaning 'to turn toward'. Separately, the abbreviation 'A.D.' comes from Latin 'Anno Domini' meaning 'in the year of (our) Lord'.

'advertisement' developed from Latin 'advertere' → Old French/advertissement → Middle English 'advertisement', and in modern English it was commonly shortened in speech and writing to 'ad'. 'Anno Domini' was used in Medieval Latin date notation and was later abbreviated in English as 'A.D.' or 'AD'.

For the advertising sense, the original Latin idea was 'to turn toward' (attention); this evolved into a notice intended to attract attention to goods or services and is now commonly called an 'ad'. For the chronological sense, 'Anno Domini' originally marked years in ecclesiastical use and remains a conventional era label (though 'CE' is now also widely used).