regretful adj.

1530s, "complaint, lament," a sense now obsolete, from the verb, or from French regret, back-formation from regreter "long after, bewail" (see regret (v.)).

From 1590s as "pain or distress in the mind due to some external circumstances" (as in to look on (something) with regret); 1640s as "pain or distress in the mind at something done or left undone."  In correspondence, in declining an invitation, etc., regrets "expressions of regret, intimation of regret for inability to do something" is attested by 1851.

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.).