"long, curving blade made fast to a handle, convenient for swinging, and used in mowing or reaping," Middle English sithe, sythe, from Old English siðe, sigði, from Proto-Germanic *segitho "sickle" (source also of Middle Low German segede, Middle Dutch sichte, Old High German segensa, German Sense), from PIE root *sek- "to cut."
The sc- spelling began by early 15c. (earliest surviving use of it is in an English word in a document written in Latin), from influence of Latin scissor "carver, cutter" and scindere "to cut." Compare French scier "saw," a false spelling from sier. Since the Middle Ages, it was carried by personifications of Time and Death.
1570s, "use a scythe;" 1590s "to mow;" from scythe (n.). By 1897 as "move with the sweeping motion of one using a scythe." Related: Scythed; scything.