An English:
After the Japanese, the English are held to be the greatest sexual perverts on earth. (I am not English).
'An English' has a special meaning in a Brothel or Bawdy House. It means a spanking.
'A Japanese' can be extremely dangerous, if done incorrectly or in the wrong hands (this means YOU), and is a tying up. (I got some flak about this so note that: It is the single largest form of accidental death amoung men in the UK.)