What is the X in X-mas?
1650s, from missionary (adj.). Missionary position attested by 1963, said to have been coined by Kinsey (1948), who identified its origin in work done by Polish anthropologist Bronisław Malinowski in Melanesia in the 1920s; allegedly from the term used by South Pacific peoples to describe what Christian missionaries promoted to replace their local variations. By late 1960s it became the general term for this type of sex, formerly also known as the English-American position.
"sent on a mission," 1640s, from Modern Latin missionarius "pertaining to a mission," from Latin missionem (see mission).