dog's letter. The letter R. In Latin, as in many of the romance languages, the R is trilled similar to the growling or snarling of a dog, earning it the sobriquet, littera canina, literally dog's letter.
The OED offers a wonderful phenomenological description from Ben Johnson's English Grammar:
“R is the dog's letter, and hurreth in the sound, the tongue striking the inner palate, with a trembling about the teeth.
1. The Oxford English Dictionary Online. 2005. 3d ed. Accessed Feb 18 2010 from http://dictionary.oed.com.
2. Garg, Anu. 2010. Dog's Letter. A. Garg, ed. A Word a Day. Wordsmith.org. Accessed Feb 18 2010 from http:// wordsmith.org/ words/ dogs_letter.html.
3. The Oxford English Dictionary Online.