Some basic definitions

To English it came from old French with the same spelling — privilege.

Etymologically it’s pretty straightforward: prīvilēgium.

Or in parts: prīvus (“private”) + lēx, lēg- (“law”).

It’s meaning given as “ordinance or law against or in favor of an individual”.

Going by the Latin, the first definition that is listed on Wikipedia states:

(ecclesiastical law, now chiefly historical) An exemption from certain laws granted by the Pope. [from 8th c.]

In (modern) Roman law it has a special meaning confined to certain rules and freedoms concerning confedentiality between e.g. client and attorney. Somewhat similar to the papal law above.

Looking into older sources

Privilege was definitely not used in the 8th century for the first time. If we take a look at Cicero(106—43 BC):

ne accusati quidem, licuit tibi ferre non legem sed nefarium «privilegium», lugente senatu, maerentibus bonis omnibus, totius Italiae precibus repudiatis, oppressa captaque re publica: mihi populo Romano implorante, senatu poscente, temporibus rei publicae flagitantibus, non licuit de salute populi Romani sententiam dicere?

In a literary English translation this looks like:

Was it lawful for you to pass, I will not say a law, but a wicked «private bill», concerning the ruin of a citizen, the preserver of the republic, as all gods and men have long since agreed to call him, and, as you yourself confess, when he was not only uncondemned but even unimpeached, amid the mourning of the senate and the lamentation of all good men, rejecting the prayers of all Italy, while the republic lay oppressed and captive at your feet?

This points to more or less the modern definition and we’re even given the word law as a contextual antonym. As in it was used as, in fact, the very opposite of a law, that is — a rule for all.

Other notes and references

In Greek, it’s somewhat related: ΠΡΟΝΌΜΙΟ (ΝΌΜΟΣ = law) ; ΠΛΕΟΝΈΚΤΗΜΑ (from ΠΛΕΟΝΈΚΤΗΣ, greedy)

Other terms to look into:

  • exlex — lawless, outside of the law
  • lēgitimus — legitimate; describe a person who is considered legitimate and lawful e.g. of married parents