Etymology
penis (n.)

"the male organ of copulation," 1670s, from French pénis or directly from Latin pēnis "penis," earlier "tail," from PIE *pes-, usually said to be originally "penis" (source also of Sanskrit pasas-, Greek peos, posthe "penis," probably also Old English fæsl "progeny, offspring," Old Norse fösull, German Fasel "young of animals, brood"). But de Vaan writes that "the meaning of pēnītus ['furnished with a tail'] as well as general semantic considerations suggest that the meaning 'tail' is original, and 'penis' metaphorically derived from it." The proper plural is penes. The adjective is penial. In psychological writing, the term penis envy is attested by 1922.

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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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pencil (n.)
Origin and meaning of pencil

mid-14c., pencel, "an artist's small, fine brush of camel hair," used for painting, manuscript illustration, etc., from Old French pincel "artist's paintbrush" (13c., Modern French pinceau) and directly from Medieval Latin pincellus, from Latin penicillus "painter's brush, hair-pencil," literally "little tail," diminutive of peniculus "brush," itself a diminutive of penis "tail" (see penis).

Small brushes formerly were used for writing before modern lead or chalk pencils. Sticks of pure graphite (commonly known as black lead) were used for marking things in England from the mid-16c., and the wooden enclosure for them was developed in the same century on the Continent. This seems to have been the time the word pencil was transferred from a type of brush to "graphite writing implement." The modern clay-graphite mix was developed early 19c., and pencils of this sort were mass-produced from mid-19c. Hymen L. Lipman of Philadelphia obtained a patent for the pencil with an attached eraser in 1858.

Derogatory slang pencil-pusher "office worker" is from 1881 (pen-driver, jocular for "clerk, writer," is from 1820); pencil neck "weak person" first recorded 1973. Pencil-sharpener as a mechanical device for putting the point on a lead pencil is by 1854.

And here is a new and serviceable invention—a pencil sharpener. It is just the thing to carry in the pocket, being no larger than a lady's thimble. It sharpens a lead pencil neatly and splendidly, by means of a small blade fitted in a cap, which is turned upon the end of a pencil. A patent has been applied for. Made by Mr. W. K. Foster, of Bangor. ["The Portland Transcript," Portland, Maine, Sept. 30, 1854]
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pillock (n.)

"the penis," 1530s (mid-13c. as a surname), dialectal variant of Middle English pil-cok, pillicock "the penis" (see cock (n.3)). Meaning "stupid person" is attested by 1967.

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phallus (n.)

1610s, "an image of the penis," from Latin phallus, from Greek phallos "penis," also "carving or image of an erect penis (symbolizing the generative power in nature) used in the cult of Dionysus," from PIE *bhel-no-, from root *bhel- (2) "to blow, swell" (source also of Old Norse boli "bull," Old English bulluc "little bull," and possibly Greek phalle "whale"). Used of the penis itself (especially if erect, but often in symbolic context) by 1891 (Hargrave Jennings).

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choad (n.)

also chode, "penis," by 1968 (Zap Comix), U.S. teen slang, of unknown origin. Guesses include supposed Navajo chodis "penis" ["Cassell's Dictionary of Slang" 2005], or a supposed Hindi, Bengali or Gujarati vernacular word for "copulate" ["New Hacker's Dictionary," 1996].

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glans (n.)
head of the penis or clitoris, 1640s, from Latin glans "acorn," also used of acorn-shaped things (see gland).
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johnson (n.)
"penis," 1863, perhaps related to British slang John Thomas, which has the same meaning (1887).
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caramba 
exclamation of dismay or surprise, 1835, from Spanish, said to be a euphemism for carajo "penis," from Vulgar Latin *caraculum "little arrow."
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dickhead (n.)

"stupid, contemptible person," by 1969, from dick in the "penis" sense + head (n.).

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