Etymology
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sum (v.)
early 14c., "to count, count up, calculate, reckon," from Old French sommer "to count, add up," or directly from Medieval Latin summare, from summa (see sum (n.)). Meaning "briefly state the substance of" is first recorded 1620s (since c. 1700 usually with up). Related: Summed; summing.
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tidy (adj.)
mid-13c., "in good condition, healthy," probably originally "in season, timely, opportune, excellent" (though this sense is not attested until mid-14c.), from tide (n.) in the sense of "season, time" + -y (2). Of persons, "of neat and orderly habits," from 1706. Similar formation in Old High German zitig, German zeitig, Dutch tijdig, Danish tidig "timely," Old English tidlic "temporal," also "timely, seasonable."
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tidy (v.)
"to make neat, set in order," 1821, from tidy (adj.). Related: Tidied; tidying.
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sum (n.)
c. 1300, summe, "quantity or amount of money," from Anglo-French and Old French summe, somme "amount, total; collection; essential point; summing up, conclusion" (13c., Modern French somme), from Latin summa "the top, summit; chief place, highest rank; main thing, chief point, essence, gist; an amount (of money)," noun use (via phrases such as summa pars, summa res) of fem. of summus "highest, uppermost," from PIE *sup-mos-, suffixed form of root *uper "over."

The sense development from "highest" to "total number, the whole" probably is via the Roman custom of adding up a stack of figures from the bottom and writing the sum at the top, rather than at the bottom as now (compare the bottom line).

General sense of "numerical quantity" of anything, "a total number" is from late 14c. Meaning "essence of a writing or speech" also is attested from mid-14c. Meaning "aggregate of two or more numbers" is from early 15c.; sense of "arithmetical problem to be solved" is from 1803. Sum-total is attested from late 14c., from Medieval Latin summa totalis.
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sum- 
assimilated form of sub- before -m-.
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dim sum (n.)

"Chinese cuisine prepared as bite-sized portions served in small steamer baskets or on small plates," 1948, from Cantonese tim sam (Chinese dianxin) "appetizer," said to mean literally "touch the heart." Ayto ("Diner's Dictionary") gives the elements as tim "dot" + sam "heart."

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cogito ergo sum 

Latin phrase, literally "I think, therefore I am;" the starting point of Cartesian philosophy (see Cartesian), from cogito, first person singular present indicative active of cogitare "to think" (see cogitation) + ergo "therefore" (see ergo) + sum, first person singular present indicative of esse "to be" (from PIE root *es- "to be").

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mod (n.1)
"tidy, sophisticated teen" (usually contrasted with rocker), 1960, slang shortening of modern.
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neatnik (n.)
"excessively tidy person," 1959, from neat (adj.) with a punning play on beatnik.
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titivate (v.)
1805, perhaps from tidy, "with a quasi-Latin ending" [OED] as in cultivate.
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