The adverb cap-a-pie (in which pie is pronounced as the word pea) means (dressed, armed) from head to foot.
It appeared as a military term in Here begynneth the first volum of sir Iohan Froyssart of the cronycles of Englande, Fraunce, Spayne, Portyngale, Scotlande, Bretayne, Flau[n]ders: and other places adioynynge (London, 1523), the translation by the English soldier and diplomat John Bourchier (circa 1467-1533), 2nd Baron Berners, of the Chroniques, a French-language prose narrative covering events during the first part of the Hundred Years’ War, from around 1326 to around 1400, by the poet and historian Jean Froissart (circa 1337-circa 1404):
They helde themselfe styll in Parys, and prouided for all thynges, as harnes and other abylments, as richely as though they had bene great lordes: and they were of harnessed men cape a pe, lyke men of armes, mo thā xxx. thousande, and as many with malles.
The second-earliest recorded instance of cap-a-pie also occurs in a military context. The English playwright and epigrammatist John Heywood (circa 1496-circa 1578) wrote, in The Spider and the Flie (London, 1556):
Ordnance of all sorts round the copweb was leyde,
And all spiders with all weapons, prest in eyde.
Daggs, handgoons, hakse, hagbussers, culuerins, slings:
Potgoons, sakirs, cannons, double and demie.
Feeld peeces, of all sewts, with al belonging things.
Byls, bowes, partisance, pikes, to push fer or nie,
And to occupy all, spiders plaste aptlie.
Ech of them: harnest meete for his properte,
The rest, all in bright harnesse capa pe.
in contemporary English:
Ordnance of all sorts round the cobweb was laid,
And all spiders with all weapons pressed in aid.
Dags¹, handguns, hakes², harquebuses, culverins, slings:
Potguns³, sakers⁴, cannons, double and demi.
Field pieces, of all suits, with all belonging things.
Bills, bows, partisans⁵, pikes, to push far or nigh,
And to occupy all, spiders placed aptly.
Each of them harnessed meet for his property,
The rest, all in bright harness cap-a-pie.
¹ dag: a kind of heavy pistol or handgun
² hake: a short firearm
³ potgun: a cannon with a separate chamber
⁴ saker: an early form of cannon smaller than a demi-culverin
⁵ partisan: a type of spear
The English term is from de cap à pied, from head to foot, used in the Middle French (i.e., the French language from about 1340 to 1611) of southern France, and in Occitan during the same period.
For example, a text in Occitan written at Albi, dating from 1428-29, says that Joan of Arc was
armada de ferblanc, tota de cap a pe.
armed with tin, entirely cap-a-pie.
In July 1538, François I, King of France, met with Charles V, the Holy Roman Emperor, at Aigues-Mortes in southern France. According to the annals of the city of Toulouse, published in 1701 probably with a modernised spelling, at this summit,
les mariniers estoient habillez de cap à pié de velours rouge.
the mariners were dressed cap-a-pie in red velvet.
And, in the essay titled Des Armes des Parthes (Of the arms of the Parthians), the French philosopher Michel Eyquem de Montaigne (1533-92)—who was from the region of Bordeaux—wrote that the Parthians
estoient armez, de cap à pied, de grosses lames de fer, rengées de tel artifice qu’à l’endroit des jointures des membres elles prestoient au mouvement. (Villey-Saulnier edition)
were armed, cap-a-pie, with great plates of iron, arranged in such a manner that at the joints of the limbs they lent themselves to the motion.
The expression used in northern French was de pied en cap, from foot to head. The original French text by Jean Froissart—who was from Valenciennes, now in northeastern France—is as follows:
Ilz se tenoient en Paris pourveuz de toutes armeures aussi bonnes et aussi riches comme se ce feussent bien grans seigneurs, et se trouvoyent arméz de pié en cap comme droittes gens d’armes, plus de XXM et plus de XXXM maillez.
Only de pied en cap has survived because the language that has become standard French is that of Île-de-France, more precisely of the cultivated Parisians.