letter是信件的意思，four-letter word可不是指“四封信的文字”，这里的four-letter是“下流的；庸俗的”的意思，four-letter word是指“粗话；猥亵语；(字母少的)粗俗下流词”的意思。

【例句】
The new financial sobriety reflected in this proposal sees risk as the cause of the current crisis, if not the nastiest four-letter word in the English language.
这一提议中反映出来的新金融理智，认为“风险”即使不是英语中最令人厌恶的粗话，也是造成当前危机的根源。

The things that stuck out were his cockiness and his four-letter words.
与众不同之处在于他的骄傲自大和满嘴脏话。