c. 1300, "body part or organ, an integral part of an animal body having a distinct function" (in plural, "the body"), from Old French membre "part, portion; topic, subject; limb, member of the body; member" (of a group, etc.)," 11c., from Latin membrum "limb, member of the body, part," probably from PIE *mems-ro, from root *mems- "flesh, meat" (source also of Sanskrit mamsam "flesh;" Greek meninx "membrane," mēros "thigh" (the "fleshy part"); Gothic mimz "flesh").
In common use, "one of the limbs or extremities." Especially "the sex organ" (c. 1300, compare Latin membrum virile, but in English originally of women as well as men). Figurative sense of "anything likened to a part of the body" is by 14c., hence "a component part of any aggregate or whole, constituent part of a complex structure, one of a number of associated parts or entities."
The transferred sense of "person belonging to a group" is attested from mid-14c., from notion of "person considered in relation to an aggregate of individuals to which he or she belongs," especially one who has united with or been formally chosen as a corporate part of an association or public body. This meaning was reinforced by, if not directly from, the use of member in Christian theology and discourse from mid-14c. for "a Christian" (a "member" of the Church as the "Body of Christ"). Meaning "one who has been elected to parliament" is from early 15c.
early 15c., "thin layer of skin or soft tissue of the body," a term in anatomy, from Latin membrana "a skin, membrane; parchment (skin prepared for writing)," from membrum "limb, member of the body" (see member). The etymological sense is "that which covers the members of the body."
plural of meninx, 1610s, "one of the three membranes enveloping the brain and spinal cord," from French meninges (1530s) or directly from medical Latin meninx, from Greek meninx (genitive meningos) "membrane," in medical Latin especially that of the brain (see member).
c. 1300, dismembren, "to cut off the limbs of," also figuratively "to scatter, disperse, divide into parts or sections so as to destroy the integrity," from Old French desmembrer (11c., Modern French démembrer), from Medieval Latin dismembrare "tear limb from limb; castrate," from Latin de "take away" (see de-) + membrum "limb" (see member). Related: Dismembered; dismembering.