Then owls and bats,
Cowls and twats,
Monks and nuns in a cloister's moods,
Adjourn to the oak-stump pantry!
"Browning had apparently somewhere come across the word twat -- which meant precisely the same then as it does now -- but pronounced it with a flat a and somehow took it to mean a piece of headgear for nuns. The verse became a source of twittering amusement for generations of schoolboys and a perennial embarrassment to their elders, but the word was never altered and Browning was allowed to live out his life in wholesome ignorance because no one could think of a suitably delicate way of explaining his mistake to him."
-- Bill Bryson,
The Mother Tongue: English and how it got that way