Ben Yagoda in Slate:
In the first episode of the Showtime series Masters of Sex, William Masters is talking to a prostitute named Betty, and notes that she faked an orgasm during sex. “Is that a common practice among prostitutes?” he asks her. “It’s a common practice amongst anyone with a twat,” replies Betty.
Clearly, two words contribute to the humor of Betty’s comeback, and the second isamongst. The show takes place in America in the 1950s, which is relevant because in that time and place virtually nobody said amongst. For the past few centuries,amongst has been a distinctly British word, though even there among is more popular. In the United States, according to the Google Books database, the last timeamongst was about as common as among was in 1720.
Curiously, however, amongst appears to be on the upswing.
More here.