What is the X in X-mas?
"pertaining to a tailor," 1807, from Modern Latin sartorius, from Late Latin sartor "tailor" (cf. French sartre "tailor"), literally "patcher, mender," from Latin sart-, past participle stem of sarcire "to patch, mend," from PIE root *serk- "to make whole." Earlier in English in same sense was Related: sartorian (1660s). Sartorius as the name of the long leg muscle is because it is used in crossing the legs to bring them into the position needed to sit like a tailor. Related: Sartorially.