libido

Libido, libidinis, f., third declension, from libet, "it pleases". Also spelt lubido in older texts, and both forms are correct.

The neutral sense is simply what you feel like doing, which is why ad libidinem means "at one's own pleasure, arbitrarily" and stands behind the modern ad lib. From there it slides two ways. Politically it names the caprice of a man who does as he likes because nobody can stop him, which is how Cicero uses it as an accusation. Morally it becomes appetite, and the Christian writers and eventually Freud left it there. Classical Latin keeps cupiditas for greed with an object and libido for wanting that has no rational one.