E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The general etymology is the French cap à pied, but the French phrase is de pied en cap.
Armed at all points exactly cap-a-pie.
Shakespeare: Hamlet, i. 2.
I am courtier, cap-a-pe.
Shakespeare: Winters Tale, iv. 3.
&since We are told that cap à pie is Old French, but it would be desirable to give a quotation from some old French author to verify this assertion. I have hunted in vain for the purpose. Again, is pie Old French for pied? This is not a usual change. The usual change would be pied into pie. The Latin might be De capte and pedem.