Etymology
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arsenic (n.)

late 14c., "yellow arsenic, arsenic trisulphide," from Old French arsenic, from Latin arsenicum, from late Greek arsenikon "arsenic" (Dioscorides; Aristotle has it as sandarake), adapted from Syriac (al) zarniqa "arsenic," from Middle Persian zarnik "gold-colored" (arsenic trisulphide has a lemon-yellow color), from Old Iranian *zarna- "golden," from PIE root *ghel- (2) "to shine," with derivatives referring to bright materials and gold.

The form of the Greek word is folk etymology, literally "masculine," from arsen "male, strong, virile" (compare arseno-koites "lying with men" in New Testament) supposedly in reference to the powerful properties of the substance. As an element, from 1812. The mineral (as opposed to the element) is properly orpiment, from Latin auri pigmentum, so called because it was used to make golden dyes. Related: Arsenical.

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member (n.)

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.

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cock (n.3)

"penis," 1610s, but certainly older and suggested in word-play from at least 15c.; also compare pillicock "penis," attested from early 14c. (as pilkoc, found in an Anglo-Irish manuscript known as "The Kildare Lyrics," in a poem beginning "Elde makiþ me," complaining of the effects of old age: Y ne mai no more of loue done; Mi pilkoc pisseþ on mi schone), also attested from 12c. as a surname (Johanne Pilecoc, 1199:  Hugonem Pillok, 1256; there is also an Agnes Pillock). Also compare Middle English fide-cok "penis" (late 15c.), from fid "a peg or plug."  

The male of the domestic fowl (along with the bull) has been associated in many lands since ancient times with male vigor and especially the membrum virile, but the exact connection is not clear (the cock actually has no penis) unless it be his role as fertilizer of the domestic hens, and there may be some influence from cock (n.2) in the "tap" sense.

The slang word has led to an avoidance of cock in the literal sense via the euphemistic rooster. Murray, in the original OED entry (1893) called it "The current name among the people, but, pudoris causa, not admissible in polite speech or literature; in scientific language the Latin is used" (the Latin word is penis). Avoidance of it also may have helped haystack replace haycock and vane displace weather-cock. Louisa May Alcott's father, the reformer and educator Amos Bronson Alcott, was born Alcox, but changed his name.

Cock-teaser, cock-sucker emerge into print in 1891 in Farmer and Henley ("Slang and Its Analogues").

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