pro tanto

1785, "two-wheeled carriage pulled by horses harnessed one behind the other" (instead of side-by-side), jocular use of Latin tandem "at length (of time), at last, so much," from tam "so" (from PIE *tam-, adverbial form of demonstrative pronoun root *-to-; see -th (1)) + demonstrative suffix -dem. "Probably first in university use" [Century Dictionary].

Transferred by 1884 to "a bicycle with two seats" (tandem-bicycle). In English as an adverb ("one behind the other, in single file") from 1795; as an adjective ("having one before the other") from 1801.

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.