There are lots of forms of this expression: upsidaisy, upsa-daisy, upsy-daisy, and oops-a-daisy, variously hyphenated on the rare occasions they turn up in print. These days, it’s just a nonsense word. It’s said to a child as encouragement to get up again after falling over, or when somebody is picking it up. Though the one thing most versions have in common is a reference to a daisy, a flower is not involved.
The common origin of all of these is up-a-daisy, dating from the early eighteenth-century. An even earlier version is the English dialect up-a-day. This is just as nonsensical a phrase, but it does show that the final part of the modern expression is actually a corruption of day.
Its history is closely bound up with lackadaisical, which started out as the cry alack-a-day!, “shame or reproach to the day!” (that it should have brought this upon me), but which by the eighteenth century had turned into lackadaisy.
Alack-a-day! was once a passionate and heartfelt cry, but it degenerated over time into a flabby exclamation of unease over some minor upset. It seems to have provided the model for up-a-day, originally a dialect term that eventually made it back into mainstream English, albeit in modulated and variable form.