Rueful means showing regret in a wry good-humoured way. A rueful smile, rueful about it, ruefully admitted.
The good humour is essential, since a rueful smile accepts the fault and laughs at it. The rue in it is the old verb meaning to regret, which survives in you will rue the day.
1adjectiveruefulShowing regret in a wry good-humoured way.
“"I should have listened to you," he said with a rueful laugh.”
rueful is a quality.
From rue, the old verb meaning to regret, from Old English hreowan.
Middle English
reuful
English, today
rueful
From Middle English reuful. From Middle English ruful, rewful. by surface analysis, en + rue + ful.
Etymology from Wiktionary, CC BY-SA