While some altruists will happily lend a hand without expecting anything in return, most of the world runs on the idea that you should be compensated in some way for your goods and services.

That’s quid pro quo, a Latin phrase which literally means “something for something.” In many cases, one of those “somethings” refers to money—you pay for concert tickets, your company pays you to teach your boss how to open a PDF, etc. However, quid pro quo also applies to plenty of situations in which no money is involved. Maybe your roommate agreed to lend you her favorite sweater if you promised to wash her dishes for a month. Or perhaps, in return for walking your neighbor’s dog while he was on vacation, he gave you his HBO login credentials.

No matter the circumstances, any deal in which you give something and you get something falls under the category of quid pro quo. According to The Law Dictionary, “it is nothing more than the mutual consideration which passes between the parties to a contract, and which renders it valid and binding.” In other words, if everyone on both sides understands the expectation that something will be given in return for a good or service, your contract is valid.

Based on that definition, quid pro quo hinges on transparency; all parties must understand that there’s an exchange being made. However, this wasn’t always the case. As the Columbia Journalism Review reports, Merriam-Webster’s dictionary entry states that quid pro quo was used in 16th-century apothecaries to denote when one medicine had been substituted for another, “whether intentionally (and sometimes fraudulently) or accidentally.”

So, if you were an unlucky peasant with a sore throat, it’s possible your herbal remedy could’ve been swapped out with something less effective—or even dangerous. Though Merriam-Webster doesn’t offer any specific examples of how or why this happened, it definitely seems like it would have been all too easy to “accidentally” poison your enemies during that time.

Just a few decades later, the term had gained enough popularity that people were using it for less injurious instances, much like we do today.

[h/t Columbia Journalism Review]