English is a bizarre language. No where is this rendered more apparent than in the faces of students who are strangers to it. I had a Swiss foreign exchange student in my English class one year. He spoke French, German and Italian fluently, and his English grammar was perfect. You would have taken him for a native English speaker too, had he not made the occasional idiomatic blunder:
"I need an umbrella. The dogs and cats are watering," he said one day.
"You mean 'it's raining cats and dogs,'" I offered.
He frowned and put the phrase into his translator. "Either way, it makes no sense. Where does that come from?"
I didn't know. It was a phrase I had used all my life, and I had no idea where it came from. A little research revealed the following gems from our language:
"Raining cats and dogs"
The phrase goes at least as far back as 1651, where it appeared in a collection of poems. As for why we say 'cats and dogs'? One particularly disgusting theory is in these less-than-hygienic times that a hard rain would wash dead animals into the streets. More likely explanations are that the animals in question are corruptions of the Greek cata doxa, meaning "contrary to experience or belief," or the Latin catadupa, meaning cataract or waterfall.
'Saved by the bell'
For most of us, it conjures up images of the popular after school show. However, the phrase may have a far darker origin. In the days before medical science became more science and less superstition, the possibility of being buried alive was a very real fear for many. One invention purported to prevent that included a string that ran from the deceased's finger all the way up to a bell above ground that could be rang should the dearly departed find themselves very much alive and six feet under. A more likely theory is that it originates in boxing, where a boxer is spared losing by the ringing of the bell at the end of the round.
'Let the cat out of the bag'
Back to those finicky felines: why do we call it 'letting the cat out of the bag' when we divulge a secret? This phrase might go back to livestock fraud, in which an unwitting buyer purchased a litter of piglets only to discover, once the bag was opened and the transaction complete, that they had been swapped out for a litter of worthless kittens. This seems to defy logic, though: to confuse mewing and oinking and the weight and feel of either animal simply because they're bagged seems a little ridiculous. Another theory refers to the 'cat o'nine tails,' a knotted cord kept on board ships to punish errant sailors. It stretches the bounds of belief, perhaps, but if it had been kept in a bag and used to punish the loose-tonged, perhaps the idiom makes a little more sense.
The origins of most idioms are lost to time, unfortunately, but it's fun to think about how our wacky mother tongue developed over time.