1610s, "to sympathize as brothers," from French fraterniser, from Medieval Latin fraternizare, from Latin fraternus "brotherly" (see fraternity). Military sense of "cultivate friendship with enemy troops" is from 1897 (used in World War I with reference to the Christmas Truce). Used oddly in World War II armed forces jargon to mean "have sex with women from enemy countries" as a violation of military discipline.
A piece of frat, Wren-language for any attractive young woman — ex-enemy — in occupied territory. [John Irving, "Royal Navalese," 1946]
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.
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Definitions of fraternise
be on friendly terms with someone, as if with a brother, especially with an enemy;