Etymology
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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 "law," 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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in (adj.)
"that is within, internal," 1590s, from in (adv.). Sense of "holding power" (the in party) first recorded c. 1600; that of "exclusive" (the in-crowd, an in-joke) is from 1907 (in-group); that of "stylish, fashionable" (the in thing) is from 1960.
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-in (1)
the adverb in attached to a verb as a word-forming element, by 1960, abstracted from sit-in, which is attested from 1941 in reference to protests and 1937 in reference to labor union actions (which probably was influenced by sit-down strike) but was popularized in reference to civil disobedience protests aimed at segregated lunch counters. As a word-forming element at first of other types of protests, extended by 1965 to any sort of communal gathering (such as love-in, 1967).
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-in (2)
word-forming element in chemistry, usually indicating a neutral substance, antibiotic, vitamin, or hormone; a modification and specialized use of -ine (2).
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in- (1)
Origin and meaning of in-
word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
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in (adv., prep.)

a Middle English merger of Old English in (prep.) "in, into, upon, on, at, among; about, during;" and Old English inne (adv.) "within, inside," from Proto-Germanic *in (source also of Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE root *en "in." The simpler form took on both senses in Middle English.

Sense distinction between in and on is from later Middle English, and nuances in use of in and at still distinguish British and American English (in school/at school). Sometimes in Middle English shortened to i.

The noun sense of "influence, access (to power or authorities)," as in have an in with, is first recorded 1929 in American English. to be in for it "certain to meet with something unpleasant" is from 1690s. To be in with "on friendly terms with" is from 1670s. Ins and outs "intricacies, complications of an action or course" is from 1660s. In-and-out (n.) "copulation" is attested from 1610s.

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in- (2)
Origin and meaning of in-

element meaning "into, in, on, upon" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "in," from PIE root *en "in."

In Old French (and hence in Middle English) this often became en-, which in English had a strong tendency to revert to Latin in-, but not always, which accounts for pairs such as enquire/inquire. There was a native form, which in West Saxon usually appeared as on- (as in Old English onliehtan "to enlighten"), and some of those verbs survived into Middle English (such as inwrite "to inscribe"), but all now seem to be extinct.

Not related to in- (1) "not," which also was a common prefix in Latin, causing confusion: to the Romans impressus could mean "pressed" or "unpressed;" inaudire meant "to hear," but inauditus meant "unheard of;" in Late Latin investigabilis could mean "that may be searched into" or "that cannot be searched into." Latin invocatus was "uncalled, uninvited," but invocare was "to call, appeal to."

The trouble has continued in English; the hesitation over what is meant by inflammable being a commonly cited example. Implume (1610s) meant "to feather," but implumed (c. 1600) meant "unfeathered." Impliable can mean "capable of being implied" (1865) or "inflexible" (1734). Impartible in 17c. could mean "incapable of being divided" or "capable of being imparted." Impassionate can be "free from passion" or it can mean "strongly stirred by passion." Inanimate (adj.) is "lifeless," but Donne uses inanimate (v.) to mean "infuse with life or vigor." Irruption is "a breaking in," but irruptible is "unbreakable."

In addition to improve "use to one's profit," Middle English also had a verb improve meaning "to disprove" (15c.). To inculpate is "to accuse," but inculpable means "not culpable, free from blame." Infestive has meant "troublesome, annoying" (1560s, from infest) and "not festive" (1620s). In Middle English inflexible could mean "incapable of being bent" or "capable of being swayed or moved." In 17c., informed could mean "current in information," formed, animated," or "unformed, formless" ("This was an awkward use" [OED]). Inhabited has meant "dwelt in" (1560s) and "uninhabited" (1610s); inhabitable likewise has been used on opposite senses, a confusion that goes back to Late Latin.

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mother-ship (n.)

"large ship or craft escorting or having charge of a number of other, usually smaller, craft," 1890, from mother (n.1) + ship (n.).

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mother-of-pearl (n.)

"nacreous inner layer of the shell of various bivalve mollusks," c. 1500, translating Medieval Latin mater perlarum, with the first element perhaps connected in popular imagination with obsolete mother (n.2) "dregs." Compare Italian madreperla, French mère-perle, Dutch parelmoer, German Perlmutter, Danish perlemor. It is the stuff of pearls but in a layer instead of a mass.

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Mother Goose 
probably a translation of mid-17c. French contes de ma mère l'oye, which meant "fairy tales." The phrase appeared on the frontispiece of Charles Perrault's 1697 collection of eight fairy tales ("Contes du Temps Passé"), which was translated in English 1729 as "Mother Goose's Tales", and a very popular collection of traditional nursery rhymes published by John Newbery c. 1765 was called "Mother Goose's Melody." Her own biographical story is no earlier than 1806.
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