We know from the Bible that fornication is a sin:
Acts 15:20 But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.
1st Corinthians 6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
1st Corinthians 7:2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
1st Thessalonians 4:3 For this is the will of God, even your sanctification, that ye should abstain from fornication:
At present, the Catholic Church believes that premarital sex is fornication:
2353 Fornication is carnal union between an unmarried man and an unmarried woman. It is gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children. Moreover, it is a grave scandal when there is corruption of the young.
But has this always been the case? According to this answer:
Fornication has changed its meaning since 1611, so reading its definition in a modern English dictionary does little good. In 1611, fornication meant prostitution (as abundantly proven ad infinitum on the "goldenrule" website) and was a perfect translation of "porneia", which is the activity of porné, or prostitutes.
So a key question here is how has the word fornication changed its meaning in the course of history.
Is there any evidence to support the theory that perhaps at some point in its history, fornication did not mean premarital sex for the Catholic Church?