Douglas Harper's Etymology Dictionary
The story is an old one, told under other names throughout Europe, of a poor boy who sends a cat he had bought for a penny as his stake in a trading voyage; the captain sells it on his behalf for a fortune to a foreign king whose palace is overrun by rats. The hero devotes part of his windfall to charity, which may be why the legend attached in England since 16c. to Sir Richard Whittington (d.1423), three times Lord Mayor of London, who died childless and devoted large sums in his will to churches, almshouses, and St. Bartholomew's Hospital.
Usage examples of "dick whittington".
On [ the station platform a statue of a Windrush boy, like a latterday Dick Whittington - bandanna around his I neck, tiny bag at his feet - stood with arms folded, one I foot bent back against the wall, ignored by the trendy new Brixtonites pushing and shoving to get on the j train with their Gucci briefcases.
Like Dick Whittington and the Count of St Germaine, Inspectre Hovis had come to Brentford with only one thought in his mind.
Alice in Wonderland or Dick Whittington, Dirk lent a whole new meaning to the word, and the most aggravating part of all was the fact that, try as Ben might, he could not decipher what it was that the beast was about!