As the dreadful story of the Ebola virus in Africa unfolds, and with it the parallel story of the panicked response to Ebola in the U.S., the word quarantine is much in the news. The stories explain that the quarantine for Ebola is 21 days. But now look at NOAD2 on the word:
quarantine noun a state, period, or place of isolation in which people or animals that have arrived from elsewhere or been exposed to infectious or contagious disease are placed: many animals die in quarantine.
verb [with obj.] impose such isolation on (a person, animal, or place); put in quarantine.
ORIGIN mid 17th cent.: from Italian quarantina ‘forty days,’ from quaranta ‘forty.’
and note the origin, involving the Italian word for ‘forty’. We have here a straightforward case in which morphological material from the etymological source is still visible in the word, yet its current use no longer respects the semantics of the source. I’ll call such words decimators, after one famous English example that has led peevers to seethe in word rage at an offense to etymology.
If you take etymology dead seriously, then referring to a 21-day isolation period as a quarantine is just wrong.