Etymology
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multi- 

before vowels mult-, word-forming element meaning "many, many times, much," from combining form of Latin multus "much, many," from PIE *ml-to-, from root *mel- (2) "strong, great, numerous." It was much-used in forming Latin compounds in classical times and after (such as multianimis "having much courage," multibibus "much-drinking," multicomus "having much hair," multiloquus "talkative"). Many English words that use it (multinational, etc.) are 20c. coinages.

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-plus 

word-forming element, Latin -plus "-fold." Watkins derives it from *-plo-, combining form of PIE root *pel- (2) "to fold" and makes it cognate with Old English -feald, Greek -paltos, -plos. But de Vaan connects it to PIE root *pele- (1) that yielded words meaning "much, many, more" and is the source of poly-.

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-y (3)
suffix in pet proper names (such as Johnny, Kitty), first recorded in Scottish c. 1400; according to OED it became frequent in English 15c.-16c. Extension to surnames seems to date from c. 1940. Use with common nouns seems to have begun in Scottish with laddie (1546) and become popular in English due to Burns' poems, but the same formation appears to be represented much earlier in baby and puppy.
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-th (1)

word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha- (cognates: Gothic -da, -ta, Old High German -do, -to, Old Norse -di, -ti), from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins].

Cognate with Sanskrit thah, Greek -tos, Latin -tus; Sanskrit ta-, Lithuanian and Old Church Slavonic to, Greek to "the," Latin talis "such;" Greek tēlikos "so old, of such an age," Old Church Slavonic toli "so, to such a degree," toliku "so much," Russian toliko "only;" also see -ed.

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-ful 
word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).
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-s (1)

suffix forming almost all Modern English plural forms of nouns, gradually extended in Middle English as -es from Old English -as, the nominative plural and accusative plural ending of certain "strong" masculine nouns (such as dæg "day," nominative/accusative plural dagas "days"). The commonest Germanic declension, traceable back to the original PIE inflection system, it is also the source of the Dutch -s plurals and (by rhotacism) Scandinavian -r plurals (such as Swedish dagar).

Much more uniform today than originally; Old English also had a numerous category of "weak" nouns that formed their plurals in -an, and other strong nouns that formed plurals with -u. Quirk and Wrenn, in their Old English grammar, estimate that 45 percent of the nouns a student will encounter will be masculine, nearly four-fifths of them with genitive singular -es and nominative/accusative plural in -as. Less than half, but still the largest chunk.

The triumphs of -'s possessives and -s plurals represent common patterns in language: using only a handful of suffixes to do many jobs (such as -ing), and the most common variant squeezing out the competition. To further muddy the waters, it's been extended in slang since 1936 to singulars (such as ducks,sweets,babes) as an affectionate or diminutive suffix.

Old English single-syllable collectives (sheep, folk) as well as weights, measures, and units of time did not use -s. The use of it in these cases began in Middle English, but the older custom is preserved in many traditional dialects (ten pound of butter; more than seven year ago; etc.).

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