Wiz, I did find the expression in “Cassell’s Dictionary of Slang” and Partridge’s “Dictionary of Catch Phrases,” however, with two somewhat different definitions, and no particular enlightening derivation. The phrase is given in these sources as “She’ is the cat’s mother (or grandmother),” along with the variant “Who’s ‘she’ – the cat’s mother (or grandmother)?”
Cassell’s Jonathon Green (also the Wordwizard of ‘Ask the Wordwizard’) says that the expression since the 1950s has been a response to the question ‘Who are you?’ when the question is considered impertinent and or over intrusive. This is ‘as I recall experiencing it,’ said Green, who is British, in his1998 Wordwizard response. So, from this and the British quotes below, the expression appears to be popular in Britain as well as Australia (although not with Green’s definition), but I have never heard it in the U.S. and can’t comment on other English-speaking countries. Green (in Cassell’s) continues with a question mark on the derivation of his particular meaning and says: “the (middle class) admonition to a child talking of ‘she,’ when describing a women, who ought to be ‘Mrs. X’ or ‘Miss Y’: “She’ is the cat’s mother.” And, thus, Green seems to be saying that the older meaning (the one you refer to) is passé and implies that it has been supplanted by his newer one. However, according to Partridge, the OED, and a sampling of the 124 Google hits for this phrase, I would tend not to agree with him.
Eric Partridge in his ‘A Dictionary of Catch Phrases’ (1977) and in his later ‘Shorter Dictionary of Catch Phrases’ (1994) compiled by Fergusson from the work of Partridge & Beale, makes no mention of Green’s newer definition. They say that “ ‘She’ is the cat’s mother (or grandmother)” dates from the mid-19th century or earlier and is “one of two or three best-known of the domestic catch phrases” – that says it’s pretty damn popular – which is usually addressed to a child by a parent who refers to his or her (or any other [[adult]] female) as ‘she.’ But “by 1960 slightly obsolete.” The variant “Who’s ‘she’ – the cat’s mother (or grandmother)?” dates from the late 19th century.
The OED seems to agree with Partridge’s assessment: “In the catch-phrase’ “who's she—the cat's mother?” and variants, said to one (especially a child) who uses the pronoun of the third person singular impolitely or with inadequate reference.”
<1897 “Don't call your mamma ‘she’. ‘SHE’ IS THE CAT.” The Beth Book’ by Sarah Grand, xx. page 204> [[British writer]]
<1913 “‘WHO’S SHE? demanded Nurse. ‘SHE’S THE CAT’S MOTHER.”—‘Sinister Street’ by C. Mackenzie, I, i. page 9> [[British writer]]
<1934 “Nonny (‘with a jerk toward Elizabeth’): Perhaps she’d play. Elizabeth: In my young days I was taught that ‘SHE’ WAS THE CAT’S GRANDMOTHER.”—‘Touch Wood’ (play) by Dodie Smith, II, iv> [[exchange between Nonny, a little girl, and Elizabeth, a spinster [[now, very un-P.C.]], aged thirty-eight ]], [[British writer]]
<1949 “‘She said so.’ Jane looked superior. ‘SHE, my boy, IS THE CAT’S MOTHER.’”—‘Painted Garden’ by Noel Streatfeild, ix. page 105> [[British writer]]
<1959 “To one who keeps saying ‘she’ in an impolite manner the reproof is: ‘WHO’S _SHE_, THE CAT’S MOTHER?’”—‘The Lore & Language of School Children’ by I. & P. Opie, iii. page 52> [[British writers]]
<1972 “WHO’S SHE? THE CAT’S GRANDMOTHER?—‘Nanny Says’ by Casson & Grenfell, page 21> [[British writers]]
<2003 “In school I was taught never to say she. / To always use a person’s given name / “SHE IS THE CAT’S MOTHER.”—‘A Picture of Dawn’ (poem) by mle>
Of course, none of the above specifically addresses your question of the phrase’s derivation. Well, my mother used to tell me every time I used to say ‘hey’ in her presence, that ‘Hey is for horses.’ In the same spirit, what could a parent (or other) say to a child when they didn’t like them referring to an adult as ‘she,’ particularly when that adult was herself. So, they had to come up with something that said ‘she’ is/means blah, blah, blah. Imagining that the child WAS referring to his own mother as ‘she,’ it is not hard to visualize that the mother, in disgust, might come up with something like ‘she’ is the mother of some lower creature. And what better lower creature, than one the child would be most familiar with such as a house dog or cat (e.g. rat, mouse, or possibly horse, cow, pig, goat might have been close – but no cigar!)? So, by executive decision of mothers of old, cat it was!
Ken – December 26, 2004
Reply from Ken Greenwald (Fort Collins, CO - U.S.A.)