[fawr-let-er, fohr-] 
| 1. | any of a number of short words, usually of four letters, considered offensive or vulgar because of their reference to excrement or sex. |
| 2. | any word, typically of four letters, that represents something forbidden, disliked, or regarded with extreme distaste: In the dieter's vocabulary, “cake” is a four-letter word. |
| four-let·ter word (fôr'lět'ər, fōr'-) n. Any of several short English words generally regarded as vulgar or obscene. |
four-letter word
Any of several short English words that are generally regarded as vulgar or obscene. For example, No four-letter words are permitted in this classroom. This expression is applied mostly to words describing excretory or sexual functions. [First half of 1900s]