This is pretty much a nonsense word that exists in a wide variety of forms and spellings dating back to the mid-19th century. It’s usually used when jumping up or surmounting some obstacle. From C. Clough Robinson’s 1862 The Dialect of Leeds and Its Neighbourhood:
Upsa daesy! a common ejaculation when a child, in play, is assisted in a spring-leap from the ground.
This form stems from an older variant of the same meaning, up-a-daisy. This term dates at least 1711, appearing in Swift’s Journal to Stella of that year:
Come stand away, let me rise...Is there a good fire?—So—up a-dazy.
The up- element is obvious, it refers to the action of jumping or climbing. the -a-daisy element is s bit more mysterious. This second portion of the word is likely simply nonsense syllables, akin to the interjections lackadaisy and alack-a-day, which despite being very different in meaning are strikingly similar in form and in part of speech.
Upsydaisy is spelled in a number of different ways. Other variants include: upsidaisy, oops-a-daisy, and whoops-a-daisy.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2013, by David Wilton