A milliner is a person (generally a woman) who makes or sells women’s hats.

But a Milliner was originally a native or inhabitant of Milan, a city in northern Italy. The word is first recorded in this sense in an Act of Parliament in 1449:

That every Venician, Italian, Januey, Florentyn, Milener, Lucan, Cateloner, Albertyns, Lumbard, Hansers, Pruciers, beyng Merchants or Factours, and all other Merchants straungiers, borne oute of youre said Lordshippes, Duchies and Isles, and dwellyng within this youre Royalme, or shall dwell duryng the said Graunte, paye to you oure Soveraigne Lord a Subsidie, that is to say, everych of theyme vi s̃. viii d̃.; and their Clerkes, everych of theym xx d̃.

According to The Merriam-Webster New Book of Word Histories (1991), 16th-century England witnessed a vogue for finery from Milan. Milan bonnets, Milan gloves, Milan jewellery, Milan needles, Milan point lace, Milan ribbons—all represented the best in Renaissance finery. The purveyors of these luxury goods imported from Milan were called milliners—although only a few of them were, in fact, from Milan.

The word is first recorded in this sense in the Privy Purse expenses of King Henry VIII. The following dates from March 1530:

It̃m the xxvij daye paied to the Mylloner for certeyne cappes trymmed and garnissed withe botons of golde  iij li. ix s̃.

And the following expense was recorded in November 1531:

It̃m the iijᵈᵉ daye paied to xp̃ofer mylloner for ij myllain bonettes for marke and the two guilliams  xvj s̃.

The word was extended to all sellers of imported fancy wares, accessories and articles of (essentially female) apparel. The following is from A Compendious or Briefe Examination of Certayne ordinary Complaints of diuers of our Countrymen in these our Dayes (1581):

Yee must consider three sortes of occupacions: one that carrieth out the treasure, the second sorte, that as it carrieth none forth of the countrey, so it bringeth none in, but that it getteth it spendeth in the countrey, the third bringeth in treasure to the countrey. Of the first sorte are Vintners, Milleners, Haberdashers, these Galley men, Mercers, Fustian sellers, Grocers, and Pothecaries that selleth vs any wares made beyond the sea, for they doe but exhaust the treasure of the Realme.

The word also designated a foreign merchant installed in Britain and specialising in fancy accessories. In A Quip for an Vpstart Courtier: Or, A quaint dispute between Veluet breeches and Cloth-breeches. Wherein is plainely set downe the disorders in all Estates and Trades (1592), the English author and playwright Robert Greene (1558-92) wrote:

But I pray you what bee those two honest men? quoth the Grocer, the one a Dutchman and a Shoomaker, the other a Frenchman & a Millainer in Sant Martins, and sels Shirts, Bands, Bracelets, Iewels, and such prety toies for Gentlewomen: oh they be of Ueluet-bréeches acquaintance, vpstarts as well as hee, that haue brought with them pride and abuses into England, and first to the Millainer. What toies deuiseth he to féede the humor of the vpstart Gentleman withall, and of fond Gentlewomen, such fans, such ouches [= clasps], such brooches, such bracelets, such graundcies [= garlands], such periwigs, such paintings, such ruffes, and cuffs, as hath almost made England as full of proud foppries as Tire & Sydon were. There is no Seamster can make a band or a shirt so well as his wife: and why forsooth? because the filthy queane weares a craunce [= a garland], and is a French woman forsooth. Where as our English women of the Exchange, are both better workwomen, and will affoord a better peniworth.

The meaning of milliner seems to have gradually changed. In Ὴγεμὼν είς τὰς γλῶσσας; id est, Ductor in linguas, The guide into tongues (1617), the English lexicographer John Minsheu (circa 1559-1627) defined it as a dealer in small articles appertaining to dress, such as thread, tape and ribbons—and gave an erroneous Latin origin:

An Habberdasher of small wares […] In London also called a Millenier, à Lat. mille. i. a thousand, as one hauing a thousand small wares to sell.

The word began to be applied to women and used in its current sense in the early 18th century. This is first recorded in The Guardian of Tuesday 1st September 1713:

The Milliner must be thoroughly versed in Physiognomy; in the Choice of Ribbons she must have a particular regard to the Complexion, and must ever be mindful to cut the Head-dress to the Dimensions of the Face. When she meets with a Countenance of large Diameter, she must draw the Dress forward to the Face, and let the Lace encroach a little upon the Cheek, which casts an agreeable Shade, and takes off from its Masculine Figure: the little Oval Face requires the diminutive Commode, just on the tip of the Crown of the Head; she must have a regard to the several Ages of Women; the Head-dress must give the Mother a more sedate Mein than the Virgin; and Age must not be made ridiculous with the flaunting Airs of Youth. There is a Beauty that is peculiar to the several Stages of Life, and as much Propriety must be observed in the Dress of the Old, as the Young.

But it was apparently not until the 19th century that milliner was reserved for a person who makes or sells women’s hats.