Prepared has its roots way back in Proto-Indo-European (the common tongue of the ancestors of all modern Indian and European languages) which had much fewer words than modern day languages, these words usually relating to communication essential for survival; run, eat, fire etc. The ancestorial word here is ‘per’ (the same per as we now see in as per usual etc) which the proto-Indo-European people used to describe the concept of ‘In front of’ which has, in modern times become ‘Pre’and is commonly used as a prefix. We have the Roman’s the thank for the slight rearrangement of the letters.
The other part of the word; ‘pare’ has its roots in the same proto-indo-European word ‘per’ or rather a version or it ‘pere’, which meant ‘To bring forward’ or ‘to produce’. The Roman’s evolved the word to ‘parare’ meaning ‘to make ready’, in the 1300s the French dropped the final ‘e’ so the word became ‘parer’ and then when the word reached English, we dropped the final ‘r’, just lazy translating really.