Mad, bad and dangerous to know

Lady Caroline Lamb’s description of Byron, of course: it’s a very famous phrase, but surprisingly hard to pin down. Where does it actually come from?

Lamb claimed she wrote the phrase (with dashes ‘mad—bad—and dangerous to know’) in her private journal for 1812; but nobody has actually seen it there—according to Paul Douglass, Lady Caroline Lamb; a biography [(Palgrave 2004), 104] ‘the phase may have been created later’. The actual source for the quotation is Lady Morgan's Memoirs (W H Allen 1862) [ii: 202]. Morgan quotes a letter from Lamb, many years after the actual encounter:

Lady Westmoreland knew him [Byron, of course] in Italy. She took on her to present him. The women suffocated him. I heard nothing of him, till one day Rogers (for he, Moore, and Spencer, were all my lovers, and wrote me up to the skies — I was in the clouds) — Rogers said, 'You should know the new poet/ and he offered me the MS. of "Childe Harold " to read. I read it, and that was enough. Rogers said, ' he has a club foot, and bites his nails.' I said, ' If he was ugly as AEsop I must know him.' I was one night at Lady Westmoreland's; the women were all throwing their heads at him. Lady Westmoreland led me up to him. I looked earnestly at him, and turned on my heel. My opinion, in my journal was, 'mad — bad — and dangerous to know.'

It’s a great phrase: the opening heavy stressed spondee underlined by its own rhyme, and followed by an iamb, and then three skipping unstressed syllables ending in a final stress (technically this last foot is a quartus paeon). It has the prosodic virtue of a kind of unspooling or unwinding rhythm, mimicking a sense of firmness giving way, before the irresistible Byronic charm. A great phrase: probably too good to have been anything other than an afterthought.