Etymology
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rat fink (n.)
also ratfink, 1963, teen slang, see rat (n.) + fink (n.). Popularized by, and perhaps coined by, U.S. custom car builder Ed "Big Daddy" Roth (1932-2001), who made a hot-rod comic character of it, supposedly to lampoon Mickey Mouse.
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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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