littera canina

• (Classical) IPA^(key): /ˈlit.te.ra kaˈniː.na/, [ˈlɪt.tɛ.ra kaˈniː.na]
• (Ecclesiastical) IPA^(key): /ˈlit.te.ra kaˈni.na/, [ˈlit.te.ra kaˈniː.na]

littera canīna f (genitive litterae canīnae); first declension

1. the letter R (literally dog's letter, after its trilled pronunciation in Latin)
   • Persius, Satires 1.108-110:

     Vide sis ne maiorum tibi forte limina frigescant: sonat hic de nare canina littera.

     Take care the thresholds of the great don’t grow chilly towards you: this is where you’ll hear the snarl of a dog’s rrrr.

First declension.
First/second declension.