From Latin 'ante' (before), with a mysterious parasitic '-t' that English added, just as it did to 'tyrant' and 'peasant.'
Belonging to the very distant past; very old; of or relating to a period of history before the fall of the Western Roman Empire (476 CE).
From Old French 'ancien' (old, original, former), from Vulgar Latin *anteānus (from before), from Latin 'ante' (before), from PIE *h₂ent- (front, forehead). The English form added an excrescent '-t' by the fifteenth century, paralleling the development in 'tyrant,' 'peasant,' and 'pageant.' The word replaced the native Old English 'eald' (old) in formal registers and became the standard adjective for the distant
The '-t' at the end of 'ancient' is a mystery addition — it does not exist in the French source 'ancien' or the Latin root 'ante.' English added this parasitic '-t' (called an excrescent consonant) in the fifteenth century, the same way it added one to 'tyrant' (from French 'tyran'), 'peasant' (from French 'paisant'), and 'pageant.'