Chaotic and disorderly; in jumbled confusion.
Reduplicated phrases are those that use the partial repetition of a word, often a nonsense word, for verbal effect. 'Higgledy-piggledy' is one of a number of such phrases that refer to chaos and disorder. Other examples are 'helter-skelter', 'harum-scarum', pell-mell', 'raggle-taggle', hobson-jobson' and 'hurly-burly'. Why reduplication, especially of words beginning with 'h', suggests jumble and disorder isn't clear.
Most reduplicated terms involve the rhyming of words of two syllables - hanky-panky, namby-pamby, mumbo-jumbo and so on. 'Higgledy-piggledy' is an unusual example that uses three-syllable words. In fact, it's a little more unusual still - it's an example of a metrical form called a 'double dactyl'. A dactyl is a three-syllable word with the stress on the first syllable and, not surprisingly, a double dactyl is a word made from two dactyls put together. Examples of these are 'inconsequentially' and 'idiosyncrasy'. 'Higgledy-piggledy' is considered such a good example of a double dactyl that it has given its name to a form of structured, some might say tortured, poetic verse that uses double dactyls. I'll spare you a reprint of one of those here; they aren't at the apex of the poet's art.
The first time that 'higgledy-piggledy' appears in print is in the first edition of John Florio's English/Italian dictionary A Worlde of Wordes, 1598:
Snatchingly, higledi-pigledie, shiftingly.
The jury is out as to whether the expression derives as a reference to pigs, but there's certainly a pretty good case to be made for a porcine origin. The variant form of the phrase, 'higly-pigly', although not found in print until 1664, seems to suggest that 17th century authors linked the phrase to pigs. If anything epitomises 'higgledy-piggledy' it's a herd of pigs. If I said I could actually prove that the person who coined 'higgledy-piggledy' had pigs in mind I would be telling porkies, but it seems highly likely.
See other reduplicated phrases.