missionary (n.)

"one who is sent on a mission, person sent by ecclesiastical authority to labor for the propagation of the faith in a place where it has no indigenous organization," 1650s, from missionary (adj.).

The phrase missionary position for "sexual intercourse arrangement in which the couple lies face to face with the woman underneath the man" is 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 the late 1960s it became the general term for this type of sex, formerly also was known as the English-American position.

"relating to or pertaining to a mission, sent on a mission," especially a Christian mission, 1640s, from Modern Latin missionarius "pertaining to a mission," from Latin missionem (see mission).