rueful adj.

"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.).

word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).

c. 1200, reufulliche, reufulike; from c. 1300 as rufully, reufulli; see rueful + -ly (2). The oldest sense, now obsolete, is "pitiably, lamentably." The meaning "mournfully, dolefully, in a sorrowful manner" is from c. 1300.