Pretentiously self-important, haughty or pompous.
Many dictionaries also give a second meaning, that is, given to frivolity, silliness or riotousness. That was the original meaning of this term, but has now almost completely died out. Our view of what is hoity-toity now is defined by the 'looking down the nose' manner adopted by characters like Lady Bracknell, as performed by Dame Edith Evans, in the stage and film versions of Oscar Wilde's The Importance of Being Earnest.
These days we hardly expect to hear lager louts described as hoity-toity. The two meanings of the term aren't as far apart as it might seem though and one seems to have migrated from the other. The frivolousness/riotousness meaning was first recorded in Sir Roger L'Estrange's 1668 translation of The visions of Don Francisco de Quevedo Villegas:
"The Widows I observ'd ... Chanting and Jigging to every Tune they heard, and all upon the Hoyty-Toyty, like mad Wenches of Fifteen."
The later meaning isn't seen until around mid to late 18th century and is recorded in O'Keefe's Fontainebleau in 1784:
"My mother ... was a fine lady, all upon the hoity-toities, and so, good for nothing."
As with many reduplicated phrases, one word carries an existing meaning and the other is present for emphasis. In this case the earlier meaning of the term came from the word hoit. This is a now defunct verb meaning to indulge in riotous, noisy mirth. That in turn was formed from hoyden - a boorish clown or rude boisterous girl.
The change from one meaning to the other seems to be due to the pronunciation of hoity as heighty and the subsequent allusion to highness or haughtiness. Two 18th century dictionaries give intermediate forms:
B.E's A new dictionary of the terms ancient and modern of the canting crew, circa 1700 - "Hightetity, a Ramp or Rude Girl."
Francis Grose's A classical of the vulgar tongue, 1785 "Heighty toity, a hoydon, or romping girl."
See other reduplicated phrases.
See also - hoi polloi.