word-forming element used to make verbs, Middle English -isen, from Old French -iser/-izer, from Late Latin -izare, from Greek -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached.
The variation of -ize and -ise began in Old French and Middle English, perhaps aided by a few words (such as surprise, see below) where the ending is French or Latin, not Greek. With the classical revival, English partially reverted to the correct Greek -z- spelling from late 16c. But the 1694 edition of the authoritative French Academy dictionary standardized the spellings as -s-, which influenced English.
In Britain, despite the opposition to it (at least formerly) of OED, Encyclopaedia Britannica, the Times of London, and Fowler, -ise remains dominant. Fowler thinks this is to avoid the difficulty of remembering the short list of common words not from Greek which must be spelled with an -s- (such as advertise, devise, surprise). American English has always favored -ize. The spelling variation involves about 200 English verbs.
chastise v.
"to inflict pain upon to punish and recall to duty, to punish for the purpose of correcting or reclaiming," c. 1300, chastisen, from Old French chastiier "to warn, advise, instruct; chastise, admonish; punish; dominate, tame" (12c., Modern French châtier), from Latin castigare "to set or keep right, to reprove, chasten, to punish," literally "to make pure" (see castigate). Or perhaps from Middle English chastien (see chasten) + -ise, though this would be early for such a native formation. The form of the modern word "is not easily accounted for" [OED]. Related: Chastised; chastising.
He alone may chastise who loves. [Rabindranath Tagore, "The Crescent Moon," 1913]