In the Early Modern period, "scythe" was actually spelled "sithe". The spelling changed to resemble a (false) classical etymology.[1] A similar thing happened to the words "island", "ache" and "tongue", which were originally spelled "iland", "ake" and "tung".

See also "some" (originally "sum"), "friend" (originally "frend"), "delight" (originally "delite") and "could" (originally "coude").

[1] - https://en.wikipedia.org/wiki/Scythe