Any of several short English words generally regarded as obscene or offensive (1920s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with 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