YOU CAN’T SQUEEZE BLOOD FROM A TURNIP
Meaning: You cannot get something from a person, especially money, that they don't have. You can only get what people are willing or able to give. A turnip cannot be coaxed, squeezed, or cajoled into producing blood. All efforts at obtaining blood from this vegetable will be futile
Origin: Unknown, but thought to originate in the Bible, with the story about Cain and Abel in Genesis 4? "Abel kept flocks (a shepherd/rancher), and Cain worked the soil (a farmer). In the course of time Cain brought some of the fruits of the soil as an offering to the Lord. But Abel brought fat portions from some of the firstborn of his flock (a meat offering requiring bloodshed). The Lord looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor." (NIV)
In the Old Testament God required a blood offering for sin. Hence, you can't get blood out of a turnip
Another reference says "blood from a turnip" was used by Frederick Marryat, "apparently borrowing an expression from folklore," in "Japhet in Search of a Father" in 1836: There's no getting blood out of a turnip."