"pair of shears of medium or small size," late 14c., sisoures, also cisours, sesours, cisurs, etc., from Old French cisoires (plural) "shears," from Vulgar Latin *cisoria (plural) "cutting instrument," from *cisus (in compounds such as Latin excisus, past participle of excidere "to cut out"), ultimately from Latin caedere "to cut" (from PIE root *kae-id- "to strike").
The spelling was highly uncertain before 20c. The forms with sc- are from 16c., by influence of Medieval Latin scissor "tailor," in classical Latin "carver, cutter," from past-participle stem of unrelated scindere "to split."
Usually with pair of (attested from c. 1400) when indication of just one is required, but a singular form without the -s occasionally was used (cysowre, mid-15c., but Middle English Compendium reports that is "only in glossaries"). In Scotland, shears (the native word) answers for all sizes, according to OED; but in England generally that word is used only for those too large to be worked by one hand. Sense in wrestling, "a grip with the legs or ankles," is by 1904. In reference to a type of swimming kick, from 1902 (the image itself is from 1880s). Oh scissors! was a 19c. exclamation of impatience or disgust (1843).