Etymology
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loch (n.)

late 14c., from Gaelic loch "lake, lake-like body," including the narrow, nearly land-locked arms of the sea found in the glacier-scoured landscape of west Scotland; cognate with Old Irish loch "body of water, lake," Breton lagen, Anglo-Irish lough, Latin lacus (see lake (n.1)). "The word was adopted in ONorthumbrian as luh" [OED]. The diminutive form is lochan.

The phrase Loch Ness monster is attested by 1934, the thing itself under slightly different names from 1933. The loch is named for the river Ness that flows out of it at Inverness; the river name is probably from an Old Celtic word meaning "roaring one."

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Nessie 

colloquial name of the "Loch Ness monster," by 1936; see loch.

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lough (n.)
"a lake, pool," early 14c., Anglo-Celtic, representing a northern form of Irish and Gaelic loch, Welsh llwch, from PIE *laku- (see lake (n.1), and compare loch).
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ch 

digraph used in Old French for the "tsh" sound. In some French dialects, including that of Paris (but not that of Picardy), Latin ca- became French "tsha." This was introduced to English after the Norman Conquest, in words borrowed from Old French such as chaste, charity, chief (adj.). Under French influence, -ch- also was inserted into Anglo-Saxon words that had the same sound (such as bleach, chest, church) which in Old English still was written with a simple -c-, and into those that had formerly been spelled with a -c- and pronounced "k" such as chin and much.

As French evolved, the "t" sound dropped out of -ch-, so in later loan-words from French -ch- has only the sound "sh-" (chauffeur, machine (n.), chivalry, etc.).

It turns up as well in words from classical languages (chaos, echo, etc.). Most uses of -ch- in Roman Latin were in words from Greek, which in Greek would be pronounced correctly as /k/ + /h/, as in modern blockhead, but most Romans would have said merely /k/, and this was the regular pronunciation in English. Before c. 1500 such words were regularly spelled with a -c- (Crist, cronicle, scoole), but Modern English has preserved or restored the etymological spelling in most of them (chemical, chorus, monarch). 

Sometimes ch- is written to keep -c- hard before a front vowel, as still in modern Italian. In some languages (Welsh, Spanish, Czech) ch- can be treated as a separate letter and words in it are alphabetized after -c- (or, in Czech and Slovak, after -h-). The sound also is heard in words from more distant languages (as in cheetah, chintz), and the digraph also is used to represent the sound in Scottish loch.

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trough (n.)
Old English trog "wooden vessel, tray, hollow vessel, canoe," from Proto-Germanic *trugaz (source also of Old Frisian, Old Saxon, Old Norse trog, Middle Dutch troch, Dutch trog, Old High German troc, German trog), from PIE *dru-ko-, from root *deru- "be firm, solid, steadfast," with specialized senses "wood, tree" and derivatives referring to objects made of wood. Originally pronounced in English with a hard -gh- (as in Scottish loch); pronunciation shifted to "-ff," but spelling remained.
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cough (v.)

"a violent, noisy effort to expel air from the lungs," early 14c., coughen, probably in Old English but not recorded, from Proto-Germanic *kokh-(source also of Middle Dutch kochen, Middle High German kuchen), with the rough "kh" of German or of Scottish loch. Onomatopoeic. Related: Coughed; coughing.

As a noun from c. 1300, "single act of coughing." As "illness or other condition that affects the sufferer with frequent coughs or fits of coughing," by 1742. Cough-drops attested by 1829; cough-medicine by 1828. To cough up "to present, hand over" is from 1894.

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lochia (n.)
"discharge from the uterus after childbirth," 1680s, Modern Latin, from Greek lokhia "childbirth," neuter plural of lokhios "pertaining to childbirth," from lokhos "a lying in, childbirth," also, "an ambush," from PIE root *legh- "to lie down, lay." Related: Lochial. Greek Lokhia also was an epithet or surname of Artemis in her aspect as protectress of women in childbirth; in this case it is the fem. of the adjective lokhios.
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hole (n.)

Old English hol (adj.) "hollow, concave;" as a noun, "hollow place; cave; orifice; perforation," from Proto-Germanic *hulan (source also of Old Saxon, Old Frisian, Old High German hol, Middle Dutch hool, Old Norse holr, German hohl "hollow," Gothic us-hulon "to hollow out"), from PIE root *kel- (1) "to cover, conceal, save." As an adjective, it has been displaced by hollow, which in Old English was only a noun, meaning "excavated habitation of certain wild animals."

As a contemptuous word for "small dingy lodging or abode" it is attested from 1610s. Meaning "a fix, scrape, mess" is from 1760. Obscene slang use for "vulva" is implied from mid-14c. Golfing hole-in-one is from 1914; as a verbal phrase from 1913. To need (something) like a hole in the head, applied to something useless or detrimental, first recorded 1944 in entertainment publications, probably a translation of a Yiddish expression such as ich darf es vi a loch in kop.

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lake (n.1)
"body of water surrounded by land and filling a depression or basin," early 12c., from Old French lack (12c., Modern French lac) and directly from Latin lacus "pond, pool, lake," also "basin, tank, reservoir" (related to lacuna "hole, pit"), from PIE *laku- "body of water, lake, sea" (source also of Greek lakkos "pit, tank, pond," Old Church Slavonic loky "pool, puddle, cistern," Old Irish loch "lake, pond"). The common notion is "basin."

There was a Germanic form of the PIE root which yielded Old Norse lögr "sea flood, water," Old English lacu "stream, pool, pond," lagu "sea flood, water, extent of the sea," leccan "to moisten" (see leak (v.)). In Middle English, lake, as a descendant of the Old English word, also could mean "stream; river gully; ditch; marsh; grave; pit of hell," and this might have influenced the form of the borrowed word.
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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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