Lewis & Short

R, r, indecl. n. or (sc. littera) f.

  1. I. The seventeenth letter of the Latin alphabet, which derives its form from the Greek P, but is not, like that, aspirated. Thus Burrus, arrabo, were originally written for Πύρρος, ἀρραβών. In words borrowed from the Greek, an h was subsequently appended to the r, as a sign of the spiritus asper. On account of its vibratory sound, resembling the snarling of a dog, r is called by Persius littera canina, Sat. 1, 109; cf. Lucil. ap. Charis. p. 100 P.
  2. II. In many words, r medial and final (but not initial) represents an original s. Tradition ascribes the introduction of r for s to Appius Claudius Caecus, consul 446 and 457 A. U. C., or to L. Papirius Crassus, consul 417 A. U. C., Dig. 1, 2, 2, § 36; Cic. Fam. 9, 21, 2. Examples of a change of s into r are: asa, lases, plusima, meliosem, meliosibus, foedesum, Fusius, Papisius, Valesius, fusvos, janitos, into ara, lares, plurima, meliorem, melioribus, foederum, Furius, Papirius, Valerius, furvus, janitor; heri (compared with hesternus and the Greek χθές); so, too, dirimo is formed from dis-emo. Cf. Varr. L. L. 7, § 26 Müll.; Cic. l. l.; Quint. 1, 4, 13; Ter. Scaur. p. 2252 and 2258 P.; Fest. s. v. Aureliam, p. 20; R pro S, p. 134; pignosa, p. 198. Both sounds have maintained their place in some substantives of the third declension ending in or or os (arbor, color, honor, labor, lepor, etc., and also arbos, colos, honos, labos, lepos, etc.); so in quaeso, quaesumus, also written quaero, quaerimus; cf. nasus and naris, pulvis and pulver, etc.
    The converse change of an original r into s appears very doubtful. Forms like hesternus (from heri), festus (also feriae), ustum (from uro), etc., indicate rather an original s, when compared with arbustum also arboretum, and majusculus also major.
    For the relation of the r to d and l, v. D and L.
  3. III. R is assimilated,
        1. a. Most freq. before l: libellus, tenellus, intellego, pellicio, from liber, tener, inter-lego, per-lacio, v. the art. per.
        2. b. Before s: dossuarius, from dorsum.
  4. IV. R is elided in pejero (from perjuro), and in the forms crebesco, rubesco, susum, also written crebresco, rubresco, sursum, etc.
  5. V. As an abbreviation, R. signifies Romanus, also Rufus, recte, reficiendum, regnum, ripa, et mult. al.; R.P. respublica; R.R. rationes relatae (cf. Fest. p. 228 Müll.).