Latin grew out of the tongue that was originally brought by the Indo-European people who migrated to eastern and southern Italy around 1193 BC shortly after the end of the Trojan War. At first the residents of the city of Rome and the region surrounding them spoke this language, then as their mighty empire evolved, Latin gradually expanded to the remaining sector around the western Mediterranean Sea. Even though it was the Roman Empire that was the main impetus behind the expansion and popularity of the Latin dialect, it also happened to be a wonderful language in its own right. It shaped brilliant literature, playing a big part in the expansion of Italian, French, Romanian, Portuguese, and Spanish, greatly influencing the English language. Latin is still a useful tool for life in the 20th century, many Latin tags or phrases have been borrowed from their words making their way into common usage in modern English then passing into the language at large. Idioms such as bona fides,magnum opus, modus operandi, per annum, prima facie, and as in this instance quid pro quo.
This phrase for “trade off”,qui pro quo means something for something; one thing given in return for another of similar value; as, The negotiator demanded a quid pro quo. The Latin noun and its meaning has been used in English since 1591 to describe equal exchange or substitution, as in I think it should be quid pro quo - you wash the car and I'll take you to the movies. The plural forms of this phrase are quid pro quos or quids pro quo.
At times, a single Latin tag can convey a little more than what can be found in English. "You scratch my back and I’ll scratch yours" doesn’t sound like an appropriate phrase when used by a head of state, so the diplomat may say quid pro quo and feel satisfied. Quid means what, pro, for and quo, what; the general picture is "something for something." In the mid-sixteenth century, apothecaries employed the term quid pro quo to refer to a medical substance that was substituted for another. Thus, the concept of exchange stuck, leading to the present sense of one favor given in trade for another.
In the world of contracts a quid pro quo is required because without a consideration they would become unilateral agreements. In the securities industry institutional patrons present orders to brokerage houses as a quid pro quo for in-depth research. Bribery can also be considered a form of a "quid pro quo payment" for a service, often an official act, such as having papers stamped for approval.
As a semantic enigma Brewster's dictionary implies that quid,a slang term for a one pound sterling probably comes from 'quid pro quo', an equivalent amount for something, and also suggests that it originally referred to a sovereign.
"Quid" has been used as slang for "pound" since the late 17th century, but no one really knows why. It may be that "quid" was adopted as a bit of clever slang based on its Latin meaning of "what," perhaps as a shortened form of an oblique slang phrase such as "what one needs" (i.e., money). Or it may be that it comes from a misunderstanding (or humorous spin on) the phrase "quid pro quo" (as in "Here's your quo, where's my quid?"). Personally, I lean toward the second theory, but we may never know for sure.
One could find further evidence for this opinion because when it comes to quid pro quo, just as pound notes tender the statement "promise to pay the bearer on demand the sum" this may indicate that it may be "cashed" in, hence perhaps the quid comes from this idea of a 'like for like' exchange.
Sources:
Christine's Latin Webpage:
members.tripod.com/~Aquabrat/index.html
Guardian Unlimited | Notes and Queries:
www.guardian.co.uk/notesandqueries/ query
The Tribune - Windows - Roots:
www.tribuneindia.com/2002/20020720/windows/roots.htm
Xrefer:www.Xrefer.com