I love to learn new words, their derivation and changing uses. Here is an unusual one I'd never heard before, quoted from Wordsmith Anu Garg--
dog's letter (dogz LET-uhr) noun
The letter R.
[From Latin littera canina, literally dog's letter. In Latin the sound of the letter R was trilled. Think Grrr! of a snarling dog. A good example of a trilling R is none other that the Spanish word for a dog: perro.]