Etymology
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involve (v.)

late 14c., "envelop, surround; make cloudy or obscure," from Old French involver and directly from Latin involvere "envelop, surround, overwhelm," literally "roll into," from in- "in" (from PIE root *en "in") + volvere "to roll," from PIE root *wel- (3) "to turn, revolve." Mid-15c. as "concern oneself." Sense of "take in, include" first recorded c. 1600. Related: Involved; Involving.

Obscurest night involved the sky,
The Atlantic billows roared,
[Cowper, "The Castaway"]
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recommendation (n.)

early 15c., recommendacion, "action of commending oneself to another" (a sense now obsolete), from Old French recommendation (Modern French recommandation), from Medieval Latin recommendationem (nominative recommendatio), noun of action from past-participle stem of recommendare (see recommend). Meaning "act of representing in a favorable manner, act of recommending (someone or something) as worthy" is from 1570s. Letter of recommendation is from c. 1500.

Letter of recommendation, a letter given by one person to another, and addressed to a third or "to whom it may concern," in which the bearer is represented as worthy of consideration and confidence. [Century Dictionary, 1895]
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plumber (n.)

late 14c. (from c. 1100 as a surname), "a worker in any sort of lead" (roofs, gutters, pipes), from Old French plomier "lead-smelter" (Modern French plombier) and directly from Latin plumbarius "worker in lead," noun use of adjective meaning "pertaining to lead," from plumbum "lead" (see plumb (n.)). The meaning focused 19c. on "workman who installs pipes and fittings" as lead pipes for conveying water and gas became the principal concern of the trade.

In U.S. history, in the Nixon administration (1969-74), it was the name of a special unit for investigation of "leaks" of government secrets.

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careful (adj.)

Old English cearful "mournful, sad," also "full of care or woe; anxious; full of concern" (for someone or something), thus "applying attention, painstaking, circumspect" (late Old English), the main modern sense; from care (n.) + -ful. In Middle English also "miserable, unfortunate," of persons or things; "causing fear, frightening, terrible." Careful-bed (early 14c.) was "sick-bed;" careful-day (c. 1200) was "judgment day."

Dragons dryfes doun
With kene carefull crie.
["The Wars of Alexander," c. 1400]
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yield (v.)
Old English gieldan (West Saxon), geldan (Anglian) "to pay, pay for; reward, render; worship, serve, sacrifice to" (class III strong verb; past tense geald, past participle golden), from Proto-Germanic *geldan "pay" (source also of Old Saxon geldan "to be worth," Old Norse gjaldo "to repay, return," Middle Dutch ghelden, Dutch gelden "to cost, be worth, concern," Old High German geltan, German gelten "to be worth," Gothic fra-gildan "to repay, requite"). This is from PIE *gheldh- "to pay," a root found only in Balto-Slavic and Germanic (and Old Church Slavonic žledo, Lithuanian geliuoti might be Germanic loan-words).

"[T]he only generally surviving senses on the Continent are 'to be worth; to be valid, to concern, apply to,' which are not represented at all in the English word" [OED]; sense development in English comes via use of this word to translate Latin reddere, French rendre. Sense of "give in return for labor or capital invested" is from early 14c. Intransitive sense of "give oneself up, submit, surrender (to a foe)" is from c. 1300. Related to Middle Low German and Middle Dutch gelt, Dutch geld, German Geld "money." Related: Yielded; yielding.
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bulimia (n.)
"emotional disorder consisting of food-gorging alternating with purging or fasting, accompanied by morbid concern with body weight and shape," 1976, Modern Latin, from Greek boulimia, "ravenous hunger" as a disease, literally "ox-hunger," from bou-, intensive prefix (originally from bous "ox;" from PIE root *gwou- "ox, bull, cow") + limos "hunger," from PIE *leie- "to waste away."

As a psychological disorder, technically bulemia nervosa. Englished form boulimy, bulimy was used from late 14c. in a medical sense of "morbidly ravishing hunger, disease causing the patient to have an insatiable hunger for food."
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opinion (n.)

early 14c., opinioun, "a judgment formed or a conclusion reached, especially one based on evidence that does not produce knowledge or certainty," from Old French opinion "opinion, view, judgements founded upon probabilities" (12c.), from Latin opinionem (nominative opinio) "opinion, conjecture, fancy, belief, what one thinks; appreciation, esteem," from stem of opinari "think, judge, suppose, opine," from PIE *op- (2) "to choose" (see option).

Where there is much desire to learn, there of necessity will be much arguing, much writing, many opinions; for opinion in good men is but knowledge in the making. [Milton, "Areopagitica"]

The word always has tended toward "a judgment or view regarded as influenced more by sentiment or feeling than reason." The meaning "formal statement by a judge or other professional" is from late 15c. The specific sense of "the estimate one forms of the character or qualities of persons or things" is by c. 1500. Public opinion, "the prevailing view in a given community on any matter of general interest or concern," is by 1735.

Middle English, perhaps reflecting the era's concern for obtaining knowledge through learned disputation, had opinional "characterized by likelihood rather than certainty" and opinial "based on probable but not certain evidence" (both mid-15c.).

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plight (n.2)

mid-13c., "solemn pledge, promise," usually concerning a serious matter and involving risk or loss in default, from Old English pliht "danger, risk, peril, damage," from Proto-Germanic *pleg- (source also of Old Frisian plicht "danger, concern, care," Middle Dutch, Dutch plicht "obligation, duty," Old High German pfliht, German Pflicht "obligation, duty," which is perhaps from PIE root *dlegh- "to engage oneself, be or become fixed," or it might be a substratum word. Compare Old English plihtere "look-out man at the prow of a ship," plihtlic "perilous, dangerous."

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observance (n.)

mid-13c., observaunce, "act performed in accordance with prescribed usage," especially a religious or ceremonial one; late 14c., "care, concern, act of paying attention (to something)," from Old French observance, osservance "observance, discipline," and directly from Latin observantia "act of keeping customs, attention, respect, regard, reverence," from observantem (nominative observans), present participle of observare "watch over, note, heed, look to, attend to, guard, regard, comply with," from ob "in front of, before" (see ob-) + servare "to watch, keep safe," from PIE root *ser- (1) "to protect." Observance is the attending to and carrying out of a duty or rule. Observation is watching, noticing.

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hey (interj.)

c. 1200 as a call implying challenge, rebuttal, anger, derision; variously spelled in Middle English hei, hai, ai, he, heh. Later in Middle English expressing sorrow, or concern; also a shout of encouragement to hunting dogs. Possibly a natural expression (compare Roman eho, Greek eia, German hei, Old French hay, French eh). In modern use often weakened, expressing pleasure, surprise.

Þa onswerede þe an swiðe prudeliche, `Hei! hwuch wis read of se icudd keiser!' ["St. Katherine of Alexandria," c. 1200]

In Latin, hei was a cry of grief or fear; but heia, eia was an interjection denoting joy.

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