'chastiser' ultimately derives from Old French (see 'chastier'), and further back from Latin 'castigare', where 'cast-' related to 'pure' and '-igare/ -igare' (from 'agere') related to 'to drive' or 'to do'.

'chastiser' developed via Old French 'chastier' (later Modern French 'châtier') into Middle English forms such as 'chastyse'/'chastisen' and ultimately produced the English verb 'chastise' and the agent noun 'chastiser'.

Originally the root carried senses of 'purify' or 'correct', and over time the meaning shifted toward 'to punish, rebuke, or correct (by chastisement)', giving the modern sense of someone who chastises.