the later Romans evidently found words beginning in sc-, sp-, st- difficult or unpleasant to pronounce; in Late Latin forms begin to emerge in i- (such as ispatium, ispiritu), and from 5c. this shifted to e-. The development was carried into the Romanic languages, especially Old French, and the French words were modified further after 15c. by natural loss of -s- (the suppression being marked by an acute accent on the e-), while in other cases the word was formally corrected back to the Latin spelling (for example spécial). Hence French état for Old French estat for Latin status, etc. It also affected Romanic borrowings from Germanic (such as espy, eschew).
The name was also changed by Anglo-French loss or metathesis of liquids in words containing -l-, -n-, or -r- (also evident in the derivatives of Old French Berengier "bear-spear" -- Old High German Beringar -- name of one of the paladins in the Charlemagne romances and a common given name in England 12c. and 13c., which has come down in surnames as Berringer, Bellanger, Benger, etc.). Thus Sarop- became Salop- and in the 12c. and 13c. the overwhelming spelling in government records was Salopesberie, which accounts for the abbreviation Salop for the modern county.
During all this, the Anglo-Saxon inhabitants (as opposed to the French scribes) still pronounced it properly, and regular sound evolutions probably produced a pronunciation something like Shrobesbury (which turns up on a 1327 patent roll). After a predictable -b- to -v- (a vowel in the Middle Ages) to -u- shift, the modern spelling begins to emerge 14c. and is fully established 15c.
Shrewsbury clock, for some reason, became proverbial for exactness, and thus, naturally, proverbial as indicating exaggeration of accuracy (1590s).
"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").