Etymology
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group (v.)
"form into a group or groups," 1718 (transitive), 1801 (intransitive), from group (n.). Related: Grouped; grouping.
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group (n.)

1690s, originally an art criticism term, "assemblage of figures or objects forming a harmonious whole in a painting or design," from French groupe "cluster, group" (17c.), from Italian gruppo "group, knot," which probably is, with Spanish grupo, from a Germanic source, from Proto-Germanic *kruppaz "round mass, lump," part of the general group of Germanic kr- words with the sense "rounded mass" (such as crop (n.).

Extended to "any assemblage, a number of individuals related in some way" by 1736. Meaning "pop music combo" is from 1958. As it was borrowed after the Great Vowel Shift in English, the pronunciation of the -ou- follows French rather than English models.

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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 (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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age-group (n.)
1876, originally a term in the science of demographics, from age (n.) + group (n.).
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in-flight (adj.)
also inflight, "during or within a flight," 1945, from in (prep.) + flight.
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in-store (adj.)
also instore, 1954, from in (prep.) + store (n.). In Middle English, instore was a verb meaning "to restore, renew," from Latin instaurare.
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shoo-in (n.)
"easy winner" (especially in politics), 1939, from earlier sense "horse that wins a race by pre-arrangement" (1928); the verb phrase shoo in in this sense is from 1908; from shoo (v.) + in (adv.).
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trade-in (n.)
1917, in reference to used cars, from verbal phrase, from trade (v.) + in (adv.).
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in-patient (n.)
also inpatient, "person lodged and fed, as well as treated, at a hospital or infirmary," 1760, from in (adj.) + patient (n.). As an adjective by 1890.
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