A shy or modest person.
Anyone coming across the expression 'shrinking violet' for the first time might be excused for being confused. You might wonder who Violet was and why she might be, like Alice in Wonderland, telescoping down to the size of a guinea-pig. The English language is chock-full of phrases that include people's names, for example 'For Pete's sake', 'Davy Jones' locker', 'Jimmy Riddle' and so on; and there's a whole page of 'Jacks'. By and large, with some notable exceptions like 'Sweet Fanny Adams' and 'Hobson's choice', these don't refer to real people. The violet in this case isn't an imaginary diffident woman but a flower. As for the shrinking, we don't mean reducing in size but modestly recoiling.
The Viola family of flowers includes violets and pansies, which are polychrome extroverts that seem just the opposite of shy. Their colourfulness makes them a favourite of municipal plantsmen around the world and even the horticulturally challenged amongst you must have seen them adorning park flowerbeds and traffic islands. However, the phrase 'shrinking violet' was coined in the UK before selective breeding turned the plants into images of steroidal blousiness. The native English violet (Viola odorata, known as the wood violet) is, as seems appropriate for our small green island, a reclusive and understated flower.
Fey Georgian gentlemen like Keats and Shelley were disposed to wandering around woodland composing poetry and it was a close friend of theirs who was doing just that when he gave the ground-hugging Viola odorata the name 'shrinking violet'. In a poetry magazine called The Indicator, the poet and essayist Leigh Hunt drew attention to the modest wood violet:
There was the buttercup, struggling from a white to a dirty yellow; and a faint-coloured poppy; and here and there by the thorny underwood a shrinking violet.
The expression may have originated as the lyrical name of a flower rather than a person but it is now mainly used figuratively to describe modest and introverted individuals. That figurative usage is first found in the USA. An early example of the phrase's use in print comes from the Pennsylvania newspaper The Titusville Herald, November 1870. The rather sarcastic article is about the New York businessman William Tweed, who was widely believed to have stolen large amounts of public money:
...deputations of the tax payers of New York waiting upon Mr. Tweed with the title-deeds of their mansions and the shrinking violet Tweed begging them to pardon his rosy blushes. Can it be that he is a humbug?
A search for 'shrinking violet' online these days brings up links to a weight reduction method that women may use to magically 'reduce by a dress size in one treatment', which nicely brings us back to where we started.