The phrase, “you can’t get blood from a turnip,” is one of the more colorful idioms in the English language. The meaning of the saying is that it’s impossible to produce a desired item or outcome from an object or situation that could not, in any case, provide it. In other words, this phrase is used to show that where potential doesn’t exist, none can be realized.
Some word historians claim that the phrase, “you can’t get blood from a turnip,” has evolved from similar phrases ever time. One example is the use of the idiom, “you can’t get honey from a rock,” where the rock is a classic example of something that does not yield either honey, or any other desirable food substance. In some of these cases, it seems that the phrase has been constructed partially to address ideas of alchemy, sorcery, or other magic, where some unnatural potential may have been thought to exist. More commonly, though, the phrase is a statement of common-sense, universally recognized limitations in the physical world.
In modern uses of the phrase "get blood from a turnip" is often used as an allegory for financial gain. Someone might use the it to suggest that, regardless of aggressive tactics, a company or other party may not be able to get payment of a bill from someone who is in poverty, or who does not have the discretionary capital to pay off the debt. This is a major use of the phrase as popular slang in English-speaking communities.
The origin of this phrase is not entirely clear. Some speculate that it goes all the way back to the historic Old Testament, where two sons, Cain and Abel, each provided a sacrifice to the Judeo-Christian God. One of these was in the form of an animal product, and the other one was in vegetable form. The difference between these two led to a significant antithesis that has produced a lot of commentary through the ages. Some feel that the phrase, “you can’t get blood from a turnip,” may refer to the lesser capacity of the undesired vegetable sacrifice in the story.
More recently, "you can’t get blood from a turnip," has been included in popular dictionaries of idioms, sayings, and clichés from the comprehensive lexicon of the English language. Some people still use it in various ways. It’s interesting to note that the phrase has survived, even though the turnip is not a major part of popular culinary culture in many English-speaking societies.