The idiom “throw out the baby with the bathwater” offers this advice: don’t rid yourself of something valuable in the process of getting rid of something undesirable.
The phrase has been in use in English from the late 19th century, and in German way before then. Where did it really come from?
One swirling rumor suggests that in Medieval times, shared bath water became so dirty that by the time the baby was bathed in it, the water was so dark with dirt that one risked forgetting the baby and throwing it out with the water. Obviously this source is untrue, as no one was ever so careless as to let their child drown in a murky tub of water.
The true source of the phrase does still come from the 16th century, however. Its first use occurred in the satire Appeal to Fools (Narrenbeschwörung) by Thomas Murner in 1512 and since then it has been a common German phrase. (The book used woodcut above, showing the quite literal interpretation.)
In the 19th century Thomas Carlyle translated the proverb into an essay against slavery, using the dirty water as a metaphor for slavery.
Since then, “throw out the baby with the bathwater” has been used regularly in the English language.