GNU Collaborative International Dictionary of English

Found one definition

1.       
   R . R, the eighteenth letter of the English alphabet, is a vocal consonant. It is sometimes called a semivowel, and a liquid. See Guide to Pronunciation, §§ 178, 179, and 250-254. “R is the dog's letter and hurreth in the sound.”
   B. Jonson.
   
   1913 Webster
    
   In words derived from the Greek language the letter h is generally written after r to represent the aspirated sound of the Greek ῥ, but does not affect the pronunciation of the English word, as rhapsody, rhetoric.
   1913 Webster
    The English letter derives its form from the Greek through the Latin, the Greek letter being derived from the Phœnician, which, it is believed, is ultimately of Egyptian origin. Etymologically, R is most closely related to l, s, and n; as in bandore, mandole; purple, L. purpura; E. chapter, F. chapitre, L. capitulum; E. was, were; hare, G. hase; E. order, F. ordre, L. ordo, ordinis; E. coffer, coffin.
   1913 Webster
   The three Rs, a jocose expression for reading, (w)riting, and (a)rithmetic, -- the fundamentals of an education.
   1913 Webster