Etymology
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pro rata 

"in proportion," from Medieval Latin pro rata (parte) "according to the calculated (share)," from pro "for, in accordance with" (see pro-) + rata, ablative singular of ratus, past participle of reri "to count, reckon" (see rate (n.)).

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prorate (v.)

also pro-rate, "divide or distribute proportionally, assess pro rata," 1860, American English, verb derived from Latin pro rata (parte); see pro rata. Related: Prorated; prorating.

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pro (n.1)

1866 as a shortening of professional (n.). The adjective is attested by 1915 (in golfing's pro shop, workshop run by the resident professional at a club). The use of professional in reference to prostitutes seems to have accounted for proette in sports writing for "female pro golfer" (1968).

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pro (n.2)

"a consideration or argument in favor," c. 1400, from Latin pro (prep.) "on behalf of, in place of, before, for, in exchange for, just as" (from PIE root *per- (1) "forward," hence "in front of, before, first, chief"). Pro and con is short for pro and contra (c. 1400) "for and against" (Latin pro et contra).

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pro- 

word-forming element meaning "forward, forth, toward the front" (as in proclaim, proceed); "beforehand, in advance" (prohibit, provide); "taking care of" (procure); "in place of, on behalf of" (proconsul, pronoun); from Latin pro (adv., prep.) "on behalf of, in place of, before, for, in exchange for, just as," which also was used as a first element in compounds and had a collateral form por-.

Also in some cases from cognate Greek pro "before, in front of, sooner," which also was used in Greek as a prefix (as in problem). Both the Latin and Greek words are from PIE *pro- (source also of Sanskrit pra- "before, forward, forth;" Gothic faura "before," Old English fore "before, for, on account of," fram "forward, from;" Old Irish roar "enough"), extended form of root *per- (1) "forward," hence "in front of, before, toward, near," etc.

The common modern sense of "in favor of, favoring" (pro-independence, pro-fluoridation, pro-Soviet, etc.) was not in classical Latin and is attested in English from early 19c.

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pro bono 

short for Medieval Latin pro bono publico "for the public good;" from pro (prep.) "on behalf of, for" (see pro-) + ablative of bonum "good" (see bene-).

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pro forma 

also proforma, Latin, literally "for form's sake, by way of formality;" from pro (prep.) "on behalf of" (see pro-) + formā, ablative of forma (see form (n.)). A pro forma invoice is one sent to the purchaser in advance of the ordered goods.

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pro tanto 

Latin, literally "for so much; to such an extent," from pro "for, so far as" (see pro-) + ablative singular neuter of tantus "so much," from tam "so" (see tandem).

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pro-slavery (adj.)

"favoring slavery; siding with the political interests of slaveholders," 1825, from pro- + slavery.

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