euthanize (v.)

1640s, "a gentle and easy death," from Greek euthanasia "an easy or happy death," from eu- "good" (see eu-) + thanatos "death" (see thanatology) + abstract noun ending -ia. Slightly earlier in Englished form euthanasy (1630s). Sense of "legally sanctioned mercy killing" is recorded in English by 1869.

chiefly British English spelling of euthanize; for suffix, see -ize. Related: Euthanised; euthanising.

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.