Etymology
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-itis 
word-forming element in medicine denoting "diseases characterized by inflammation" (of the specified part), Modern Latin, from Greek -itis, feminine of adjectival suffix -ites "pertaining to." Feminine because it was used with an implied nosos "disease," a feminine noun; especially in arthritis (nosos) "(disease) of the joints." Arthritis (16c.) was one of the earliest appearances of the suffix in English and from it the suffix was abstracted in other uses.
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-ian 
variant of suffix -an (q.v.), with connective -i-. From Latin -ianus, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective. In Middle English frequently it was -ien, via French.
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-gate 
suffix attached to any word to indicate "scandal involving," 1973, abstracted from Watergate, the Washington, D.C., building complex that was home of the National Headquarters of the Democratic Party when it was burglarized June 17, 1972, by operatives later found to be working for the staff and re-election campaign of U.S. President Richard Nixon.
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-some (2)
suffix added to numerals meaning "a group of (that number)," as in twosome, from pronoun use of Old English sum "some" (see some). Originally a separate word used with the genitive plural (as in sixa sum "six-some"); the inflection disappeared in Middle English and the pronoun was absorbed. Use of some with a number meaning "approximately" also was in Old English.
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-plex 
word-forming element, from Latin -plex, from PIE root *plek- "to plait." De Vaan writes, "Probably, duplex was the archetype of this category of compounds."
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-tron 
word-forming element in compounds coined in physics, "having to do with electrons or subatomic particles," 1939, abstracted unetymologically from electron (Greek -tron was an instrumentive suffix).
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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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-ate (3)

in chemistry, word-forming element used to form the names of salts from acids in -ic; from Latin -atus, -atum, suffix used in forming adjectives and thence nouns; identical with -ate (1).

The substance formed, for example, by the action of acetic acid (vinegar) on lead was described in the 18th century as plumbum acetatum, i.e. acetated lead. Acetatum was then taken as a noun meaning "the acetated (product)," i.e. acetate. [W.E. Flood, "The Origins of Chemical Names," London, 1963]
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chemo- 

before vowels chem-, word-forming element denoting "relation to chemical action or chemicals," from combining form of chemical (adj.), used to form scientific compound words from c. 1900. In 19c., chemico- was used.

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-ial 
adjectival word-forming element, variant of -al (1) with connective -i-. From Latin -ialis, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective.
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