Etymology
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-i (2)
plural suffix sometimes preserved in English for words from Latin, it is the Latin plural of nouns of the second declension (such as focus/foci, radius/radii).
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I (pron.)

12c., a shortening of Old English ic, the first person singular nominative pronoun, from Proto-Germanic *ek (source also of Old Frisian ik, Old Norse ek, Norwegian eg, Danish jeg, Old High German ih, German ich, Gothic ik), from PIE *eg- "I," nominative form of the first person singular pronoun (source also of Sanskrit aham, Hittite uk, Latin ego (source of French Je), Greek ego, Russian ja, Lithuanian ).

Reduced to i by mid-12c. in northern England, later everywhere; the form ich or ik, especially before vowels, lingered in northern England until c. 1400 and survived in southern dialects until 18c. It began to be capitalized mid-13c. to mark it as a distinct word and avoid misreading in handwritten manuscripts.

The reason for writing I is ... the orthographic habit in the middle ages of using a 'long i' (that is, j or I) whenever the letter was isolated or formed the last letter of a group; the numeral 'one' was written j or I (and three iij, etc.), just as much as the pronoun. [Otto Jespersen, "Growth and Structure of the English Language," p.233]

The dot on the "small" letter -i- began to appear in 11c. Latin manuscripts to distinguish the letter from the stroke of another letter (such as -m- or -n-). Originally a diacritic, it was reduced to a dot with the introduction of Roman type fonts. The letter -y- also was written with a top dot in Old English and early Middle English, during the centuries when -i- tended to be written with a closed loop at the top and thus was almost indistinguishable from the lower-case thorn (þ). In names of U.S. highways (by 1966) it is short for Interstate (adj.).

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-i (1)
as a termination in certain people names (Iraqi, Israeli), it represents the common Semitic national designation suffix -i.
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-i- 
a "connective" element in many words formed with Latin or Greek suffixes, now often felt as part of them (as in -iac, -iacal, -ial, -ian, -ify, -ity, etc.). Properly it forms no proper part of the suffix but is often the stem-vowel of the initial word in the Latin compounds (genial from genius), or a modified form of it. As such forms were very common, -i- was used merely connectively or euphonically in some Latin compounds (uniformis) and in later words made from Latin components in English or French (centennial, editorial).

The Greek equivalent is -o-, which also became an active connective in English, but they now are used indifferently with elements from either language.
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*dhe(i)- 

*dhē(i)-, Proto-Indo-European root meaning "to suck."

It forms all or part of: affiliate; affiliation; effeminate; effete; epithelium; fawn (n.) "young deer;" fecund; fellatio; Felicia; felicitate; felicity; Felix; female; feminine; femme; fennel; fenugreek; fetal; feticide; fetus; filial; filiation; filicide; filioque; fitz; infelicity.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dhayati "sucks," dhayah "nourishing;" Greek thēlē "mother's breast, nipple," thēlys "female, fruitful;" Latin felare "to suck," femina "woman" ("she who suckles"), felix "happy, auspicious, fruitful," fetus "offspring, pregnancy;" fecundus "fruitful, fertile, productive; rich, abundant;" Old Church Slavonic dojiti "to suckle," dojilica "nurse," deti "child;" Lithuanian dėlė "leech;" Old Prussian dadan "milk;" Gothic daddjan "to suckle;" Old Swedish dia "suckle;" Old High German tila "female breast;" Old Irish denaim "I suck," dinu "lamb."

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*po(i)- 
*pō(i)-, Proto-Indo-European root meaning "to drink."

It forms all or part of: beer; bever; beverage; bib; bibitory; bibulous; hibachi; imbibe; imbrue; pinocytosis; pirogi; poison; potable; potation; potion; symposium.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pati "drinks," panam "beverage;" Greek pinein "to drink," poton "that which one drinks," potos "drinking bout;" Latin potare "to drink," potio "a potion, a drinking," also "poisonous draught, magic potion;" Old Church Slavonic piti "to drink," pivo "beverage."
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I Ching 
1876, from Chinese, said to mean "Book of Changes."
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I-beam (n.)
1869; see beam (n.). So called for its shape. I-bar is from 1890; also I-rail (1873).
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Koh-i-noor (n.)
famous diamond, one of the British crown jewels after the annexation of Punjab in 1849, from Persian koh-i-nur, literally "mountain of light," from Persian koh "mountain" + Arabic nur "light."
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I'se 
1847 in representations of African-American vernacular, a contraction of I is (see is), irregular for I am. In Scottish and northern English, a colloquial or dialectal contraction of I shall (1796).
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