'bac' originates from French, specifically the word 'baccalauréat' (often shortened in French to 'bac').

'bac' is a clipped form of French 'baccalauréat'; the French term itself comes from Medieval Latin 'baccalaureatus' (related to 'baccalaureus'), and the short form 'bac' entered informal usage in French and was borrowed into English as slang.

Initially it referred to the full term 'baccalauréat' (the qualification/exam); over time the clipped form 'bac' has been used colloquially to mean the exam or the qualification itself and, in English contexts, has also been applied as a loanword or as an acronym (BAC) for unrelated technical terms.