Old English sorg "grief, regret, trouble, care, pain, anxiety," from Proto-Germanic *sorg- (source also of Old Saxon sorga, Old Norse sorg, Middle Dutch sorghe, Dutch zorg, Old High German soraga, German sorge, Gothic saurga), perhaps from PIE *swergh- "to worry, be sick" (source also of Sanskrit surksati "cares for," Lithuanian sergu, sirgti "to be sick," Old Church Slavonic sraga "sickness," Old Irish serg "sickness"). Not connected etymologically with sore (adj.) or sorry.
"sorrow, repentance," Middle English reue, from Old English hreow "grief, repentance, sorrow, regret, penitence," from Proto-Germanic *hrewwo "pain; sadness, regret, repentance," source also of Frisian rou, Middle Dutch rou, Dutch rouw, Old High German (h)riuwa, German reue "sorrow, regret, repentance," nouns from the root of rue (v.).
late 14c., "lamenting, complaining, giving utterance to sorrow or grief," from Old French plaintif "complaining; wretched, miserable," from plainte (see plaint). Sense of "expressive of sorrow or melancholy, mournful, sad" is recorded from 1570s. Earlier was pleintful "grievous, lamentable" (early 14c.). Related: Plaintively; plaintiveness.
c. 1200, perhaps late Old English, ruthe, "misery, sorrow, grief;" also "pity, compassion, sorrow for the misery of another" (often in have ruth, take ruth); also "remorse, repentance, regret;" from Old Norse hryggð "ruth, sorrow," from hryggr "sorrowful, grieved" (see rue (v.)) + Proto-Germanic abstract noun suffix *-itho (see -th (2)).
Or else formed in English from reuwen "to rue" on the model of true/truth, etc. The Old English word was rue (n.2).