The term derives from carnal, meaning "of the flesh", and the Biblical usage of the verb know/knew, a euphemism for sexual conduct.
One examples of this usage is in the first part of the Bible, the Book of Genesis, which describes how Adam and Eve conceived their first child: And Adam knew Eve his wife; and she conceived, and bore Cain, and said, I have gotten a man with [the help of] Jehovah.Also in Genesis is Lot's plea to the people of Sodom to whom he offered his virgin daughters, in place of his guests: And they called unto Lot, and said unto him, Where are the men that came in to thee this night?
While commonly a mere euphemism for sexual intercourse (not necessarily unlawful), different jurisdictions have defined carnal knowledge (as well as sexual intercourse) as a specific sex act such as contact between a penis and vagina, some laws elaborating this to include even "slight penile penetration of female sex organs."
Some laws do not define the term, and leave it to the courts to give it meaning, which also allows them to take into account changing community standards.
Section 18 of the Offences against the Person Act 1828 reads as follows: What shall be sufficient Proof of carnal Knowledge in the Four preceding Cases.