Etymology
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loc. cit. 
abbreviation of Latin loco citato or locus citatus "in the place (already) cited;" hence, "in the book that has been previously quoted." See locus, cite. In use in English books by 1704.
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cit (n.)

"inhabitant of a city," colloquial shortening of citizen, 1640s; especially "a London cockney," as contrasted to a country man or a gentleman, usually with some measure of opprobrium (Johnson defines it as "A pert low townsman; a pragmatical trader").

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op. cit. 

abbreviation of Latin opus citatum "the work quoted;" see opus; citatum is neuter singular past participial of citare "to call, call forward, summon" (see cite).

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locket (n.)
mid-14c., "iron cross-bar of a window," from Old French loquet "door-handle, bolt, latch, fastening" (14c.), diminutive of loc "lock, latch," from Frankish or some other Germanic source (compare Old Norse lok "fastening, lock;" see lock (n.1)). Meaning "little ornamental case with hinged cover" (containing a lock of hair, miniature portrait, etc.) first recorded 1670s. Italian lucchetto also is from Germanic.
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lock (n.2)
"tress of hair," Old English locc "lock of hair, curl" (plural loccas), from Proto-Germanic *lukkoz (source also of Old Norse lokkr, Old Saxon, Old Frisian, Dutch lok, Old High German loc, German Locke "lock of hair"), a word of uncertain origin. According to OED, perhaps from a PIE *lugnos- and related to Greek lygos "pliant twig, withe," Lithuanian lugnas "flexible" (see reluctance).
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*op- 

Proto-Indo-European root meaning "to work, produce in abundance."

It forms all or part of: cooperate; cooperation; copious; copy; cornucopia; hors d'oeuvre; inure; maneuver; manure; oeuvre; office; official; officinal; omni-; omnibus; omnium gatherum; op. cit.; opera; operate; operation; operose; optimism; optimum; opulence; opulent; opus; Oscan.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit apas- "work, religious act," apnas- "possession, property;" Hittite happina- "rich;" Avestan huapah- "doing good work, masterly;" Latin opus "a work, labor, exertion;" Greek ompne "food, corn;" Old High German uoben "to start work, to practice, to honor;" German üben "to exercise, practice;" Dutch oefenen, Old Norse æfa, Danish øve "to exercise, practice;" Old English æfnan "to perform, work, do," afol "power."

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lock (n.1)

"means of fastening," Old English loc "bolt, appliance for fastening a door, lid, etc.; barrier, enclosure; bargain, agreement, settlement, conclusion," from Proto-Germanic *lukana-, a verbal root meaning "to close" (source also of Old Frisian lok "enclosure, prison, concealed place," Old Norse lok "fastening, lock," Gothic usluks "opening," Old High German loh "dungeon," German Loch "opening, hole," Dutch luik "shutter, trapdoor").

Ordinary mechanical locks work by means of an internal bolt or bar which slides and catches in an opening made to receive it. "The great diversity of meaning in the Teut. words seems to indicate two or more independent but formally identical substantival formations from the root" [OED]. The Old English sense "barrier, enclosure" led to the specific meaning "barrier on a stream or canal" (c. 1300), and the more specific sense "gate and sluice system on a water channel used as a means of raising and lowering boats" (1570s).

From 1540s as "a fastening together," hence "a grappling in wrestling" (c. 1600). In firearms, the part of the mechanism which explodes the charge (1540s, probably so called for its resemblance to a door-latching device), hence figurative phrase lock, stock, and barrel (which add up to the whole firearm) "the whole of something" (1842). Phrase under lock and key attested from early 14c.

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pun (n.)

"a Conceit arising from the use of two Words that agree in the Sound, but differ in the Sense" [Addison]; "An expression in which the use of a word in two different applications, or the use of two different words pronounced alike or nearly alike, presents an odd or ludicrous idea" [Century Dictionary]; 1660s (first attested in Dryden), a word of uncertain origin.

Perhaps from pundigron, meaning the same thing (though attested first a few years later), itself a word of uncertain etymology, perhaps a humorous alteration of Italian puntiglio "equivocation, trivial objection," diminutive of Latin punctum "point." This is pure speculation. Punnet was another early form.

Pun was prob. one of the clipped words, such as cit, mob, nob, snob, which came into fashionable slang at or after the Restoration. [OED]

 The verb, "to make puns," also is attested from 1660s, first in Dryden. Related: Punned; punning.

At the revival of learning, and the spread of what we may term the refinement of society, punning was one of the few accomplishments at which the fine ladies and gentlemen aimed. From the twelfth to the sixteenth century, it was at its greatest height. The conversation of the witty gallants, and ladies, and even of the clowns and other inferior characters, in the comedies of Shakespeare and his contemporaries, which we may be sure was painted from the life, is full of puns and plays upon words. The unavoidable result of such an excess was a surfeit, and the consequent dégout, which lasted for more than a century. Like other diseases, it broke out again subsequently with redoubled virulence, and made great havoc in the reign of Queen Anne. [Larwood & Hotten, "The History of Signboards from the Earliest Times to the Present Day," London, 1867] 
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