Ruth

Old English hreowan (class II strong verb; past tense hreaw, past participle hrowen), "make (someone) sorry, cause (someone) to grieve, distress, affect with regret," transitive senses now obsolete, from Proto-Germanic *khrewan (which is source also of Old Frisian riowa, Middle Dutch rouwen, Old Dutch hrewan, German reuen "to sadden, cause repentance").

It is in part it has been blended with the Old English weak verb hreowian "feel pain or sorrow," and perhaps influenced by Old Norse hryggja "make sad." Both are from Proto-Germanic *khruwjan, all from PIE root *kreue- (2) "to push, strike" (see anacrusis).

The meaning "repent of, feel remorse for, feel regret for something or how it happened," is attested by c. 1200; the intransitive sense of "be sorrowful or penitent, experience grief" is recorded from 14c. Related: Rued; ruing.

King John. France, thou shalt rue this hour within this hour. ["King John," Act III, Scene 1]

suffix forming nouns of action, state, or quality from verbs or adjectives (such as depth, strength, truth), from Old English -ðu, -ð, from Proto-Germanic *-itho (cognates: Old Norse -þ, Old High German -ida, Gothic -iþa), abstract noun suffix, from PIE *-ita (cognates: Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free"). Sometimes in English reduced to -t, especially after -h- (as in height).