Origin of "old bag"?
Not so much a corruption of old hag as rhyming slang for it. The New Partridge Dictionary of Slang and Unconventional English quotes Ray Puxley, an expert on Cockney rhyming slang, as suggesting this might be the case.
old bag noun
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an unattractive or unloveable old woman, UK 1949
Disparaging; possibly a variant of OLD BAT, cognisant of OLD BAG (elderly prostitute) which itself may derive from OLD BAT. Ray Puxley, writing in 1992, suggests this may be rhyming slang, formed on 'hag'.
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an elderly, slatternly prostitute; hence pejorative for a younger prostitute
-- Julian Franklyn, A Dictionary of Rhyming Slang, 1961
The Online Etymology Dictionary says it dates from 1924 or earlier but does not give any sources.
Disparaging slang for "woman" dates from 1924 (though various specialized senses of this are much older).
I have always thought the "bag" part to be an abbreviation of "baggage", which has long been a pejorative term for a woman, and still is in many parts of the UK and particularly (in my experience) Ireland. For example, Shakespeare uses "baggage" in The Merry Wives of Windsor (Act 4 Scene 2), first published in 1602:
Out of my door, you witch, you hag, you baggage, you polecat, you runyon!
Grose's Classical Dictionary of the Vulgar Tongue (1823) confirms this sense of "baggage"
BAGGAGE. Heavy baggage; women and children. Also a familiar epithet for a woman; as, cunning baggage, wanton baggage, &c.
Whether my belief is correct or not, I have found the term in print in 1894, 30 years earlier than etymonline's claim, in a publication called **Wales; a national magazine for the English speaking parts of Wales**. The passage come in part 11 of a serialisaton of "Enoch Hughes" (originally "Enoc Huws") by Daniel Owen, translated from Welsh by the Hon. Claud Vivian. According to an Amazon reviewer, this translation was from 1892
"I should like to be a scholard, master, to be able to understand business," said Margaret, unconcernedly putting the note on the table and leaving the room.
"You are enough of a scholar for me, you old bag," said Enoch to himself, putting on his boots.