Etymology
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common law (n.)

mid-14c., "the customary and unwritten laws of England as embodied in commentaries and old cases" (see common (adj.)), as opposed to statute law. Phrase common-law marriage is attested from 1909.

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common (adj.)
Origin and meaning of common

c. 1300, "belonging to all, owned or used jointly, general, of a public nature or character," from Old French comun "common, general, free, open, public" (9c., Modern French commun), from Latin communis "in common, public, shared by all or many; general, not specific; familiar, not pretentious." This is from a reconstructed PIE compound *ko-moin-i- "held in common," compound adjective formed from *ko- "together" + *moi-n-, suffixed form of root *mei- (1) "to change, go, move," hence literally "shared by all."

The second element of the compound also is the source of Latin munia "duties, public duties, functions," those related to munia "office." Perhaps reinforced in Old French by the Germanic form of PIE *ko-moin-i- (compare German gemein, Old English gemne "common, public, general, universal;" see mean (adj.)), which came to French via Frankish.

Used disparagingly of women and criminals since c. 1300. Meaning "pertaining equally to or proceeding equally from two or more" is from c. 1400. Meaning "usual, not exceptional, of frequent occurrence" is from late 14c. Sense of "not distinguished, belonging to the general mass" is from c. 1400; of things, "ordinary, not excellent," late 14c.

Common pleas is 13c., from Anglo-French communs plets, hearing civil actions by one subject against another as opposed to pleas of the crown. Common prayer is that done in public in unity with other worshipers; contrasted with private prayer. Common stock is attested from 1888. Common speech (late 14c.) is the vernacular, as opposed to Latin. Common good (late 14c.) translates Latin bonum publicum "the common weal." The college common room (1660s) is one to which all members have common access. 

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marriage (n.)
Origin and meaning of marriage

c. 1300, mariage, "action of entering into wedlock;" also "state or condition of being husband and wife, matrimony, wedlock;" also "a union of a man and woman for life by marriage, a particular matrimonial union;" from Old French mariage "marriage; dowry" (12c.), from Vulgar Latin *maritaticum (11c.), from Latin maritatus, past participle of maritare "to wed, marry, give in marriage" (see marry (v.)). The Vulgar Latin word also is the source of Italian maritaggio, Spanish maridaje, and compare mariachi.

Meanings "the marriage vow, formal declaration or contract by which two join in wedlock;" also "a wedding, the celebration of a marriage; the marriage ceremony" are from late 14c. Figurative use (non-theological) "intimate union, a joining as if by marriage" is from late 14c.

[W]hen two people are under the influence of the most violent, most insane, most delusive, and most transient of passions, they are required to swear that they will remain in that excited, abnormal, and exhausting condition until death do them part. [G.B. Shaw, preface to "Getting Married," 1908]

Marriage counseling is recorded by that name by 1939. Marriage bed, figurative of marital intercourse generally, is attested from 1580s (bed of marriage is from early 15c.).

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common (n.)
Origin and meaning of common

c. 1300, "a fellowship or brotherhood; early 14c., "people of a community or town, freemen, citizenry;" late 15c., "land held in common," from Old French commune and Medieval Latin communia, and partly from common (adj.). Also compare commons. Latin communis "common, general" (adj.) also served as a noun meaning "common property; state, commonwealth."

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law (n.)
Origin and meaning of law

Old English lagu (plural laga, combining form lah-) "ordinance, rule prescribed by authority, regulation; district governed by the same laws;" also sometimes "right, legal privilege," from Old Norse *lagu "law," collective plural of lag "layer, measure, stroke," literally "something laid down, that which is fixed or set."

This is reconstructed to be from Proto-Germanic *lagam "put, lay" (from PIE root *legh- "to lie down, lay"). The modern word is thus a twin of lay (n.2) as "that which is set or established."

Rare in Old English, it ousted the more usual ae and also gesetnes, which also were etymologically "something placed or set." 

In physics, "a proposition which expresses the regular order of things," from 1660s. Law and order have been coupled since 1796. To lay down the law (1752) is pleonastic (the "law" in the figure is biblical law, laid down from the pulpit). Poor laws provided for the support of paupers at public expense; sumptuary laws restrained excesses in apparel, food, or luxuries.

It is more common for Indo-European languages to use different words for "a specific law" and for "law" in the general sense of "institution or body of laws," for example Latin lex "a law," ius "a right," especially "legal right, law."

Indo-European words for "a law" are most commonly from verbs for "to put, place, set, lay," such as Greek thesmos (from tithemi "to put, place"), Old English dom (from PIE *dhe- "to put, place, set"), Lithuanian įstatymas (from statyti "cause to stand, set up, establish"), Polish ustawa (from stać "stand"). Also compare Old English gesetnes (above), statute, from Latin statuere; German Gesetz "a law, statute," from Old High German gisatzida "a fixing, determination, assessment," with sezzen (modern German setzen) "to make sit, set, put."

Words for "law" in the general sense mostly mean etymologically "what is right" and often are connected with adjectives for "right" (themselves often figurative uses of words for "straight," "upright," "true," "fitting," or "usage, custom." Such are Greek nomos (as in numismatic); French droit, Spanish derecho, from Latin directus; Polish prawo, Russian pravo (from Old Church Slavonic pravŭ "straight," in the daughter languages "right"); also Old Norse rettr, Old English riht, Dutch recht, German Recht (see right (adj.1)).

[L]earn to obey good laws before you seek to alter bad ones [Ruskin, "Fors Clavigera"]
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law (v.)

1640s, "to litigate," from law (n.). Old English had lagian "make a law, ordain." Related: Lawed; lawing.

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common-sensical (adj.)

1839, from common sense, with ending as in nonsensical, etc. Common-sensible is attested from 1848.

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common sense (n.)

late 14c., originally an internal mental power supposed to unite (reduce to a common perception) the impressions conveyed by the five physical senses (Latin sensus communis, Greek koine aisthesis). Thus "ordinary understanding, without which one is foolish or insane" (1530s); the meaning "good sense" is from 1726. Also, as an adjective, common-sense "characterized by common sense" (1854).

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Parkinson's Law 

1955 (in the "Economist" of Nov. 19), named for its deviser, British historian and journalist Cyril Northcote Parkinson (1909-1993): "work expands to fill the time available for its completion."

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Boyle's law (n.)

named for Irish-born chemist and physicist Robert Boyle (1627-1691), who published it in 1662.

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