To avoid performing an ordinary social duty or otherwise excuse a breach of civility, merely say:
I'm terribly sorry; I know it's wrong of me; but honestly I can't help it. You see, I'm a bit mad. [Then add, with a half-wistful, half-triumphant air,] There's madness in my family, you know. —
E. V. Lucas, Windfall's Eve (1930)