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.
"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.)).
