'preach' originates from Latin, specifically the word 'praedicare', where 'prae-' meant 'before' and 'dicare' meant 'to proclaim or declare publicly'.

'preach' entered English via Old French 'prechier' and Middle English 'prechen', ultimately from Late Latin 'praedicare'; over time the form became the modern English 'preach'.

Initially it meant 'to proclaim publicly' (especially a religious proclamation); over time it retained the religious sense but broadened to mean 'advocate or urge' more generally, and also gained the sense of 'moralize' in a reproving way.