The verb to quarantine comes from the noun quarantine, but where does the noun come from? Turns out it can be traced all the way back to the Proto-Indo-European word for "four", kweter. This is the root of the word "four" in basically every Indo-European language, from Slavic to Sanskrit, the commonality of which is remarkable because of its infrequence. Anyway, kweter disintigrated along with PIE, and one of its remnants dragged itself into Latin quattor, still meaning "four" (another descendant was the Proto-Germanic word fedwor, which led to today's fewer). This fed into the later Latin word quadraginta, or "forty". This happily existed along until medieval scholars in the sixteenth century turned everything upside down. Being very religious, they wanted to give a Latin name to the desert that Jesus fasted in for forty days, so they decided to be completly unoriginal and just call the desert "forty". This resulted in a corruption of the Latin word and the new word, quarentyne. This obviously gave way to quarantine, and explains the etymological connection between that and, say, quarter or Spanish quarenta. Other sources will tell you that a quarantine is the amount of time sick sailors had to be isolated from society; while this may be true, it is not the origin of that.
Adam Aleksic, a leading contender for valedictorian of his high school, is a 208-month-old boy with disturbing interests in etymology, vexillology, geography, and law. Adam would like to one day visit Tajikistan and just won an essay contest on linguistics!
The Etymology Nerd