The story
Perhaps ultimately derived in some way from Vulgar Latin *cisellum (from *caesellum, from caedō), whence e.g. French cission, ciseau and other words with senses related to cutting and separating, or perhaps from the same root as scission (“division, separating”) (Latin scissus, scindō), or even a conflation of the two as in scissor, whence sciss (“cut with scissors”).
Etymology from Wiktionary, CC BY-SA 4.0.